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L.  B.   Cat.  No.  1137 


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THE  UNIVERSITY  OF  CHICAGO 
SCIENCE  SERIES 


Editorial  Committee 

ELIAKIM  HASTINGS  MOORE,  Chairman 

JOHN  MERLE  COULTER 

ROBERT  ANDREWS  MILLIKAN 


THE  UNIVERSITY  OF  CHICAGO 
SCIENCE  SERIES,  established  by  the 
Trustees  of  the  University,  owes  its  origin 
to  a  belief  that  there  should  be  a  medium  of 
publication  occupying  a  position  between  the 
technical  journals  with  their  short  articles  and 
the  elaborate  treatises  which  attempt  to  cover 
several  or  all  aspects  of  a  wide  field.  The 
volumes  of  the  series  will  differ  from  the  dis- 
cussions generally  appearing  in  technical  jour- 
nals in  that  they  will  present  the  complete  re- 
sults of  an  experiment  or  series  of  investigations 
which  previously  have  appeared  only  in  scat- 
tered articles,  if  published  at  all.  On  the 
other  hand,  they  will  differ  from  detailed 
treatises  by  confining  themselves  to  specific 
problems  of  current  interest,  and  in  presenting 
the  subject  in  as  summary  a  manner  and  with 
as  little  technical  detail  as  is  consistent  with 
sound  method.  They  will  be  written  not  only 
for  the  specialist  but  for  the  educated  layman. 


INDIVIDUALITY   IN   ORGANISMS 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


THE  BAKER  &  TAYLOR  COMPANY 
NEW  YORK 

THE  CAMBRIDGE   UNIVERSITY  PRESS 
LONDON 

THE  MARUZEN-KABUSHIKI-KAISHA 
TOKYO,  OSAKA,  KYOTO,  FUKUOKA,  SENDAI 

THE  MISSION  BOOK  COMPANY 
SHANGHAI 


INDIVIDUALITY  IN 
ORGANISMS 


Ey 

CHARLES  MANNING  CHILD 

Professor  of  Zoology,  University  of  Chicago 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


Copyright  1915  by 
The  University  of  Chicago 


All  Rights  Reserved 


Published  November  191 5 
Second  Impression  July  1924 


Composed  and  Printed  By 

The  University  of  Chicag-o  Press 

Chicago,  Illinois,  U.S.A. 


CHAPTER 

I.  The  Problem 


CONTENTS 

PAGE 


The  Characteristics  of  the  Organic  Individual;  Unity  and 
Order  in  the  Life  of  the  Individual;  Reproduction  and 
Individuation;    Metabolism  and   Protoplasm;    Terminology. 

II.  Theories  of  Organic  Individuality 21 

Theoretical  Review  and  Critique;  A  Dynamic  Conception  of 
the  Organic  Individual. 

III.  Metabolic  Gradients  in  Organisms 50 

Susceptibility  Gradients  in  Animals  and  Plants;  Further 
Physiological  Evidence  for  the  Existence  of  Metabolic  Gra- 
dients; Embryological  Evidence  for  the  Existence  of  Axial 
Metabolic  Gradients;  Developmental  Gradients  in  Agamic 
and  Experimental  Reproduction;  Conclusion. 

IV.  Physiological  Dominance  in  the  Process  of  Indi- 
viduation   88 

The  Experimental  Material;  The  Independence  of  the  Apical 
Region;  Dominance  and  Subordination  in  Experimental 
Reproduction;  The  Reconstitution  of  an  Individual  from  an 
Isolated  Piece;  Some  Modifying  and  Limiting  Factors  in 
Animal  Reconstitution;  Conclusion. 

V.  The  Range  of  Dominance,  Physiological  Isolation, 
AND  Experimental  Reproduction 127 

Experimental  Control  of  Spatial  Relations  of  Parts  and  of  the 
Range  of  Dominance;  Experimental  Obliteration  and 
Determination  of  Axial  Gradients  and  Dominance;  The 
Extension  of  Dominance  during  Development;  Experimental 
Physiological  Isolation  and  Reproduction  in  Plants;  The 
Localization  of  Experimental  Reproduction  in  Relation  to 
Different  Axes;  Conclusion. 


N 


vu 


12U06 


viii  CONTENTS 

CHAPTER  PAGE 

\T.  Discussion,  Conclusions,  and  Suggestions     .     .     .170 

The  Nature  of  Dominance;  The  Nature  of  Inhibition; 
Origin  of  MetaboHc  Gradients  and  of  Dominance;  Morpho- 
logical Differentiation  in  Relation  to  Metabolic  Rate;  The 
Fundamental  Reaction  System;  Agamic  Reproduction  in 
Relation  to  Physiological  Isolation;  Gametic  Reproduction; 
Heredity,  Evolution,  and  Other  Problems  from  the  Dynamic 
Standpoint. 

Index 209 


PREFACE 

The  present  book  is  an  attempt  to  state,  and  to 
present  some  of  the  evidence  in  favor  of,  a  conception  of 
the  nature  of  organic  individuality  which  has  gradually 
developed  in  the  mind  of  the  writer  during  the  course  of 
some  fifteen  years'  investigation  of  the  simpler  processes 
of  reproduction  and  development  in  the  lower  animals. 
In  these  forms  organic  individuality  appears  in  rela- 
tively simple  terms,  and  it  is  here  if  anywhere  that  we 
must  look  for  the  key  to  the  problem  of  individuality  in 
the  higher  animals  and  man. 

With  the  great  variety  of  facts  at  hand  and  the 
limited  space  available,  it  has  often  been  difficult  to 
decide  what  particular  points  of  the  evidence  to  include 
in  the  consideration  and  what  to  omit.  To  those 
familiar  with  biological  facts  it  will  doubtless  be  evident 
that  many  data  from  various  lines  of  investigation  have 
been  either  barely  mentioned  or  entirely  omitted. 

The  attempt  has  been  made  to  show  in  some  degree 
the  wide  range  of  applicability  of  this  conception  of 
individuality  to  various  biological  fields,  and  it  is  per- 
haps permissible  to  express  the  hope  that,  not  only  the 
physiologist  and  botanist,  but  also  the  neurologist,  the 
psychologist,  and  the  sociologist  may  find  something 
of  interest  in  it.  Chaps,  i  and  ii  are  necessarily  some- 
what abstract  and  condensed  and  may  seem  to  some 
readers  to  demand  too  extensive  a  background  of  bio- 
logical knowledge.  A  re-reading  of  these  chapters 
after  reading  chaps,  iii-vi  will  assist  in  decreasing  this 
difficulty. 

In  the  book  Senescence  and  Rejuvenescence ,  recently 
published,   the  writer  was  chiefly  concerned  with   the 

ix 


X  PREFACE 

periodic  changes  of  the  age  cycle  in  the  organic  indi- 
\ddual  as  one  aspect  of  the  life-cycle.  The  present  book 
deals  primarily  with  the  problem  of  the  nature  of  the 
unity  and  order  in  the  organism,  the  constancy  of  char- 
acter and  course  of  development,  the  maintenance  of 
individuality  in  a  changing  environment,  and  the 
processes  of  physiological  isolation,  disintegration,  and 
integration  or  individuation  in  reproduction.  The  two 
books,  concerned  as  they  are  v/ith  these  intimately 
associated  aspects  of  the  life  cycle,  are  in  many  respects 
complementary  and  together  constitute  a  presentation  of 
the  more  important  results  and  conclusions  from  the 
writer's  investigations  and  a  consideration  of  certain 
biological  problems  from  the  point  of  view  attained. 

For  permission  to  reproduce  or  to  make  drawings 
based  upon  figures  which  have  appeared  in  other  books 
acknowledgments  are  due  to  publishers  as  follows:  to 
the  University  of  Chicago  Press,  for  Figs.  i6,  17,  25-27, 
47,  48  in  part,  reproduced  from  Senescence  and  Reju- 
venescence; to  Wilhelm  Englemann,  for  Fig.  13,  drawn 
after  von  Graff's  Fig.  8,  Tafel  XVIII,  in  Monographic 
der  Turbellarien,  I,  Rhabdocoelida ;  to  Henry  Holt  &  Co., 
for  Figs.  14  and  15,  reproduced  from  Lillie,  Development 
of  the  Chick;  to  Gustav  Fischer,  for  Fig.  86,  reproduced 
from  Hildebrand,  Die  Gattung  Cyclamen;  to  B.  G. 
Teubner,  for  Fig.  92,  drawn  after  Fig.  84  in  Goebel, 
Einleitung  in  die  experimentelle  Morphologic  der  Pflanzen. 
For  certain  figures  drawn  from  botanical  preparations 
and  for  other  figures  not  original,  acknowledgments  are 
made  in  the  legends.  The  writer  is  also  deeply  indebted 
to  Dr.  C.  J.  Herrick  and  to  Dr.  W.  J.  G.  Land,  both  of 
the  University  of  Chicago,  for  reading  the  manuscript 
and  for  suggestions  and  criticisms. 

C.  M.  Child 

October,  19 15 


CHAPTER  1 
THE  PROBLEM 

The  organic  world  appears  in  the  form  of  more  or  less 
clearly  defined  and  limited  and  more  or  less  complex 
entities  which  for  lack  of  a  better  name  we  call  individ- 
uals. The  individual  is  not  necessarily  a  single  whole 
organism;  it  may  be  a  part  of  a  cell,  a  single  cell,  or  a 
many-celled  organ  or  complex  part  of  the  organism; 
or,  as  in  most  plants  and  some  of  the  lower  animals,  a 
number  of  organisms  possessing  certain  organs  or  parts 
in  common,  and  therefore  remaining  in  organic  con- 
tinuity with  each  other,  may  together  constitute  an 
individual.  In  at  least  most  organic  individuals  a 
more  or  less  orderly  series  of  changes  in  structure  and 
behavior  which  comprise  the  life-history  occur,  and  in  the 
course  of  these  changes  the  individuals  may  give  rise  to 
new  individuals  by  some  sort  of  reproductive  process. 

In  order  to  define  the  problem  of  the  organic  indi- 
vidual, it  is  necessary  to  inquire  whether  any  funda- 
mental identity  or  similarity  is  discoverable  in  all 
individuals  and  whether  the  changes  which  they  undergo 
are  subject  to  any  general  laws  which  we  can  at  present 
apprehend. 

THE   CHARACTERISTICS   OF   THE   ORGANIC   INDIVIDUAL 

The  term  ''individual,"  meaning  in  its  etymological 
sense  something  undivided  or  which  cannot  be  divided, 
is  open  to  various  objections.  Division  of  individuals 
to  form  new   individuals  is  a  characteristic  feature  of 


2  INDIVIDUALITY  IN  ORGANISMS 

living  forms,  and  many  individuals  are  made  up  of 
other  individuals  and  these  in  turn  of  others.  Divisi- 
bility is  as  characteristic  of  the  organic  individual  as 
indivisibility.  Nevertheless  individuality  is  a  very  real 
thing  in  the  organic  world.  There  may  often  be  diffi- 
culty in  determining  the  presence  or  absence  of  indi- 
viduality or  the  limits  in  space  or  time  of  particular 
individuals,  but  such  difficulties  do  not  in  the  least 
shake  our  faith  in  the  existence  of  the  individual.  What- 
ever the  anatomist  and  the  histologist  tell  us  concerning 
the  constitution  of  the  human  body,  of  hundreds  of 
organs  and  millions  of  cells,  it  is  perfectly  evident  that 
each  human  being  is  an  individual,  because  his  behavior 
proves  it=  And  the  same  is  true  for  the  single  cell  with 
its  nucleus,  cytoplasm,  centrosomes,  plastids  of  various 
kinds,  "mitochondria,"  chromosomes,  chromomeres,  etc. 
We  call  the  cell  an  individual  because  of  its  behavior. 
What  then  are  the  fundamental  characteristics  of 
this  behavior  ?  In  what  does  the  individuality  consist  ? 
In  the  first  place,  the  organic  individual  is  alive  and 
therefore  consists  essentially  of  the  complex  of  substances 
termed  in  general  protoplasm;  secondly,  it  is  more  or 
less  definitely  limited  in  size;  thirdly,  it  possesses  a 
more  or  less  definite  morphology,  a  visible  form  and 
structure,  which  is  associated  in  some  way  with  dynamic 
and  primarily  chemical  activity;  fourthly,  a  greater  or 
less  degree  of  order,  co-ordination,  correlation,  or  har- 
mony, as  it  is  variously  called,  is  perceptible  in  the  char- 
acter of  its  form  and  structure  and  in  the  dynamic 
activities  of  its  constituent  parts.  In  short,  the  organic 
individual  appears  to  be  a  unity  of  some  sort,  its  indi- 
viduality consists  primarily  in  this  unity,  and  the  process 


THE  PROBLEM  3 

of  individuation  is  the  process  of  integration  of  a  mere 
aggregation  into  such  a  unity,  for  this  unity  is  not 
simply  the  unity  of  a  chance  aggregation,  but  one  of  a 
very  particular  kind  and  highly  constant  character 
for  each  kind  of  individual.  In  all  except  the  simplest 
individuals  it  determines  a  remarkable  degree  of  uni- 
formity and  constancy,  both  in  the  spatial  relations  of 
parts  and  the  order  of  their  appearance  in  time,  and  also 
in  co-ordination  or  harmony  of  functional  relation  of 
these  parts  after  their  development. 

If  this  conception  is  correct,  the  fundamental  problem 
of  the  organic  individual  is  the  problem  of  the  nature 
of  this  unity.  The  first  step  in  consideration  of  this 
problem  is  to  inquire  whether  this  unity  is  real  or  appar- 
ent. Conceivably  it  may  be  only  an  apparent  or  pas- 
sive unity  resulting  from  a  pre-established  harmony  of 
some  kind  between  the  constituent  parts,  a  unity  like 
that  of  a  house  constructed  from  girders,  stone,  and 
other  materials,  each  part  of  which  is  measured  and  cut 
beforehand  according  to  a  definite  plan.  The  real  unity 
here  is  in  the  plan,  not  in  its  material  realization.  This 
conception  of  the  individual  leads  necessarily  to  the 
assumption  of  a  creative  entity  of  some  sort  which  con- 
trols and  orders  the  physico-chemical  organism  as  man 
controls  and  orders  the  materials  of  the  house  which  he 
builds.  This  dualistic  or  ''vitalistic"  conception  of  the 
organic  individual,  carried  to  its  logical  conclusion,  denies 
the  possibility  of  solution  of  the  problem  of  organic  indi- 
viduality by  scientific  methods.  Some  of  its  special 
forms  will  be  briefly  considered  in  another  chapter. 

On  the  other  hand,  the  unity  of  the  organic  indi- 
vidual may  be  an  active  unity  resulting  from  interactions 


4  INDIVIDUALITY  IN  ORGANISMS 

between  its  parts,  but  such  a  unity  may  again  con- 
ceivably be  the  result  of  a  pre-established  harmony  in 
construction  like  that  of  a  steam  engine,  or  it  may  be  a 
unity  which  itself  determines,  constructs,  and  har- 
monizes as  a  flowing  t^ream  sculptures  its  channel  and 
develops  a  characteristic  morphological  structure  and 
dynamic  activity  in  mutual  relation  to  each  other.' 
According  to  this  last  purely  mechanistic  conception  the 
problem  of  individuality  is  accessible  to  scientific  investi- 
gation and  may  be  solved  by  scientific  methods. 

While  it  is  impossible  to  exclude  absolutely  the 
dualistic  alternative  as  long  as  a  complete  mechanistic 
solution  of  the  problem  has  not  been  reached,  the 
advance  of  scientific  knowledge  has  resulted  in  demon- 
strating the  mechanistic  character  of  one  feature  after 
another  of  the  organism  and  in  narrowing  the  field 
within  which  vitalistic  assumptions  are  still  possible. 
We  know  that  actual  energetic  relations  do  exist  between 
the  different  parts  of  the  individual.  These  relations, 
which  are  often  called  physiological  correlation,  are  of 
various  sorts:  mechanical,  such  as  pressure  or  tension 
between  parts;  transportative,  consisting  in  the  trans- 
portation or  exchange  of  substances  between  different 
parts;  transmissive  or  conductive,  consisting  of  changes, 
impulses,  or  excitations  transmitted  or  conducted  from 
molecule  to  molecule  or  from  particle  to  particle.  Physi- 
ological correlation  of  these  different  kinds  unques- 
tionably pla^ys  a  very  important  part  in  the  unity  of 
the  individual,  and  the  only  possible  method  of  pro- 
cedure to  determine  whether  the  unity  consists  essen- 

*  Child,  "The  Regulatory  Processes  in  Organisms,"  Jour,  oj 
Morphol.,  XXII,  191 1. 


THE  PROBLEM  5 

tially  in  such  correlation  is  the  scientific  method,  that 
of  experimental  analysis  and  synthesis  of  data.  Only  in 
this  way  is  there  any  possibility  of  determining  whether 
or  not  the  mechanistic  conception  is  adequate.  My 
own  experiments,  together  with  the  experimental  and 
observational  data  already  at  hand,  point  the  way  toward 
a  conception  of  organic  unity  which  is  somewhat  different 
from  current  views,  but  still  entirely  mechanistic,  and 
which,  as  I  believe,  makes  possible  further  advance 
toward  a  solution  of  the  problem. 

UNITY   AND    ORDER   IN   THE    LIFE    OF   THE    INDIVIDUAL 

Life  in  general  consists  of  the  life-histories  of  indi- 
viduals. Individuals  arise  from  other  individuals, 
undergo  a  more  or  less  definite  and  orderly  series  of 
changes  known  as  development,  usually  reproduce  new 
individuals  in  a  more  or  less  orderly  way,  and  either 
undergo  complete  physiological  disintegration  into  new 
individuals  in  the  process  of  reproduction  or  else  finally 
lose  their  unity  by  the  cessation  of  their  activity  in 
death. 

The  definitiveness  and  constancy,  the  degree  of  order 
in  the  behavior  of  the  individual  as  regards  the  morpho- 
logical and  physiological  relations  of  its  parts  in  space 
and  the  sequence  of  the  changes  during  its  life,  must 
be  considered  as,  to  some  extent  at  least,  a  criterion  of 
the  degree  of  unity  or  individuality  which  it  possesses. 
In  the  simplest  individuals  order  is  scarcely  apparent; 
it  is  sometimes  difficult  to  determine  whether  a  particular 
aggregation  of  protoplasmic  substance  is  an  individual  in 
the  biological  sense  or  merely  an  aggregation.  Again, 
in  some  cases  a  given  order  is  local  or  temporary  and  is 


6  INDIVIDUALITY  IN  ORGANISMS 

soon  succeeded  by  another.  Each  pseudopodium  of  an 
amoeba,  for  example,  is  to  some  extent  an  individuation 
of  a  part  of  the  amoeba  protoplasm,  but  it  soon  disap- 
pears or  gives  way  to  another  individuation,  and  so  on. 
Between  such  simple  and  evanescent  individuals  as  this, 
at  the  one  extreme,  and  the  human  body  with  its  amaz- 
ingly complex  structural  and  functional  order  and  its 
relative  permanency  at  the  other,  there  are  of  course 
many  intermediate  conditions.  In  general,  organic  evo- 
lution appears  from  this  point  of  view  to  consist  in  an 
increasing  complexity  and  stability  of  order;  in  other 
words,  the  degree  of  individuation,  the  unity  of  the 
individual,  increases  in  the  course  of  evolution.  The 
series  of  changes  which  constitute  development  becomes 
more  and  more  definite,  constant,  and  complex  in  ap- 
pearance, and  the  product  of  these  changes,  the  fully 
formed  individual,  shows  an  increasing  complexity  and 
stability  of  structure  and  an  increasing  variety  and 
degree  of  interrelation  of  parts.  In  fact,  a  progressive 
morphological  and  physiological  complication  seems  to 
occur  both  in  individual  development  and  in  evolution. 
Between  the  unicellular  organism  and  the  adult  human 
being  the  difference  appears  to  be  almost  infinite,  but 
the  human  individual  is  at  the  beginning  a  single  cell 
with  much  less  complex  visible  structure  than  many 
unicellular  forms. 

The  process  of  visible  structural  complication  which 
occurs  in  both  development  and  evolution  is  commonly 
known  as  differentiation.  Different  regions  of  the  cell, 
different  cells  or  cell  groups,  become  different  from  each 
other  and  from  the  original  undifferentiated  or  so-called 
embryonic  condition.     These  differences  are  in  general 


THE  PROBLEM 


7 


brought  about  by  the  formation  and  accumulation  in  or 
about  the  cell  of  substances  not  present  in  the  undiffer- 
entiated cell.  Differentiation  is  of  course  merely  a 
visible  indication  of  differences  of  some  sort  in  physio- 
logical activity  in  different  parts,  although  physiological 
differences  may  exist  without  visible  differentiation. 
The  physiological  differences  appear  to  consist  at  least 
in  large  part  of  specialization  in  activity,  that  is,  the 
various  fundamental  activities  of  life  which  are  all 
present  to  some  degree  in  the  unspecialized  cell  or  part 
become  more  or  less  definitely  distributed  and  localized 
among  different  parts,  a  process  often  called  division  of 
labor.  Such  specialization  of  parts  is  a  characteristic 
feature  of  life  in  all  except  the  simplest  individuals,  and 
even  there  it  is  probably  present  to  some  extent. 

Physiological  specialization  and  the  differentiation 
which  may  result  from  it  occur  in  any  orderly  way,  and 
in  fact  constitute  the  fundamental  evidence  for  the 
orderly  character  of  the  individual.  The  orderly 
course  of  specialization  and  differentiation  proceeds 
very  much  as  if  there  were  underlying  it  a  plan  or  scheme 
characteristic  for  each  kind  of  individual  which  is 
worked  out  in  a  regular  constant  order,  as  the  construc- 
tion of  a  building  according  to  a  plan  follows  a  regu- 
lar course.  The  orderly  localization  of  parts  and  the 
orderly  sequence  in  their  appearance  with  reference  to 
certain  directions  in  the  developing  individual  indicate 
the  existence  of  some  sort  of  ordering  capacity  under- 
lying and  preceding  the  stages  where  the  order  becomes 
structurally  visible.  It  is  evident  that  this  underlying 
order,  plan,  or  whatever  it  may  be  that  determines  the 
developmental  and  physiological  order  in  the  individual 


8  INDIVIDUALITY  IN  ORGANISMS 

is  the  foundation  of  individual  unity  and  order,  but  in 
attempts  to  solve  the  problem  of  the  individual  this 
fact  has  not  always  been  clearly  recognized. 

This  underlying  capacity  for  unity  and  order  finds 
its  primary  expression  in  what  is  commonly  called 
the  polarity  and  S3rmmetry  of  the  individual.  These 
terms  polarity  and  symmetry  refer  to  the  fact  that  in 
the  appearance  and  maintenance  of  the  structural  and 
functional  order  in  the  individual  certain  geometrical 
relations,  characteristic  for  each  individual,  are  dis- 
tinguishable. These  relations  are  commonly  expressed 
in  terms  of  axes  or  planes.  To  say  that  an  individual 
possesses  a  polar  axis  or  a  plane  of  symmetry  is  merely 
a  convenient  way  of  stating  the  fact  that  it  is  possible  to 
conceive  as  drawn  through  the  individual  a  line  or  a 
plane  with  reference  to  which  order  is  perceptible. 
Such  a  geometrical  conception  is  an  abstraction  from 
the  fact  that  order  is  actually  perceptible  to  a  greater 
or  less  degree  in  all  directions.  It  is  merely  a  selection 
of  those  ideal  lines  or  planes  to  which  the  order  is  most 
directly,  simply,  or  permanently  related,  and  these  then 
serve  as  a  system  of  co-ordinates,  so  to  speak,  to  which 
the  order  is  conveniently  referred. 

The  geometrical  relations  of  order  differ  in  different 
individuals.  In  some  cases  the  order  is  referable  to  a 
system  of  lines  passing  through  a  common  center  and  is 
designated  as  radial  or  radiate.  In  other  cases  the 
order  is  referable  to  one  or  a  certain  number  of  axes  and 
is  therefore  an  axiate  order,  and  it  is  often  convenient 
to  refer  to  planes  instead  of  axes  of  symmetry.  In  the 
living  individuals  as  they  exist  in  nature  various  com- 
binations of  these  relations  occur.     In  the  starfish,  for 


THE  PROBLEM  g 

example,  a  high  degree  of  order  is  apparent  in  relation 
to  an  axis  drawn  through  the  center  of  the  body  vertical 
to  the  plane  in  which  the  arms  extend.  Centering  about 
this  axis  is  an  order  referable  to  radii  centering  in  this 
axis,  and  again  the  order  in  each  arm  is  referable  to  a 
plane  passed  through  the  radius  of  each  arm  and  the 
central  axis  of  the  body.  In  a  bilaterally  symmetrical 
individual,  such  as  man  and  many  animals,  the  order 
can  be  referred  to  three  axes — longitudinal,  transverse, 
and  dorso-ventral — at  right  angles  to  each  other,  or 
perhaps  better  to  a  longitudinal  axis,  and  two  planes, 
transverse  and  ventro-dorsal,  passed  at  right  angles  to 
each  other  through  this  longitudinal  axis. 

These  axes  and  planes  drawn  through  the  individual 
as  a  whole  represent  merely  the  general  plan  of  orderly 
arrangement.  Geometrical  relations  are  also  distin- 
guishable in  the  order  of  various  parts  and  organs,  and 
these  relations  do  not  necessarily  coincide  in  direction 
with  the  geometrical  scheme  of  the  whole  individual 
but  differ  from  it  in  all  conceivable  ways.  Evidently 
the  geometrical  relations  of  order  in  the  organic  indi- 
vidual, particularly  where  the  structure  is  complex,  are 
by  no -means  as  simple  as  the  general  scheme  might  seem 
to  indicate. 

The  reason  for  making  a  distinction  between  polarity 
and  symmetry  lies  in  the  fact  that  in  most  axiate 
individuals  one  axis,  the  polar  axis,  is  distinguishable 
as  the  chief  or  major  axis  of  the  body.  In  the  direction 
of  this  axis  the  specialization  and  differentiation  are 
most  marked  and  the  order  in  this  direction  is  usually 
more  conspicuous  or  more  stable  and  commonly  appears 
earlier  than  that  in  other  directions.     This  axis  is  also 


lo  INDIVIDUALITY  IN  ORGANISMS 

very  often  the  chief  direction  of  growth,  so  that  the  body 
becomes  elongated  in  this  direction  and  the  polar  axis 
becomes  the  longitudinal  axis.  In  short,  the  so-called 
polarity  of  the  individual  represents  the  direction  of  the 
chief  or  major  order,  while  the  axes  of  symmetry  repre- 
sent the  directions  of  minor  orders. 

The  two  terminal  regions  of  the  polar  axis  exhibit  in 
general  distinct  and  characteristic  differences  in  behavior 
and  structure.  In  most  plants,  in  sessile  animals,  and 
in  radially  symmetrical  forms  generally,  these  two 
regions  are  commonly  called  the  apical  and  basal  regions, 
while  in  bilaterally  symmetrical  motile  animals  they 
are  usually  known  as  anterior  and  posterior.  The  apical 
or  anterior  region  is  primarily  the  region  of  greatest 
dynamic  or  metabolic  activity  in  the  individual:  in  the 
plants  it  becomes  the  growling  tip,  the  region  of  most 
active  primary  growth,  while  in  the  animals  it  becomes 
the  most  highly  specialized  and  differentiated  region  of 
the  body,  and  in  those  forms  which  possess  a  central 
nervous  system,  including  all  except  the  simplest  animals, 
the  chief  part  of  the  central  nervous  system,  the  cephalic 
ganglion  or  brain  and  the  chief  sense  organs  usually  arise 
in  this  region;  in  other  words  it  becomes  the  head  and 
in  motile  forms  usually  precedes  in  locomotion. 

The  basal  or  posterior  end,  on  the  other  hand,  is 
primarily  the  least  active  region,  although  in  many 
forms  it  may  become  secondarily  a  region  of  growth  or 
specialized  activity  because  of  certain  changes  during 
the  life  of  the  individual  wliich  will  be  considered  later. 
In  sessile  forms  it  is  usually  the  region  of  attachment  and 
may  develop  special  organs  of  attachment,  while  in 
motile  forms  its  activity  is  more  or  less  under  the  control 


THE  PROBLEM  il 

of  the  apical  or  anterior  region.  In  fact  it  is  impossible 
to  escape  the  conclusion  that  certain  general  features 
common  to  most  or  all  axiate  individuals  are  similarly 
related  to  the  polar  apico-basal  or  antero-posterior  axis, 
as  it  is  variously  called.  As  regards  other  axes,  the 
differences  in  relation  between  them  and  the  differences 
in  behavior  and  structure  in  different  individuals 
complicate  the  matter,  but  I  shall  show  that  there  are 
good  grounds  for  believing  that  an  organic  or  physio- 
logical axis  is  fundamentally  the  same  in  all  cases, 
whether  it  is  an  axis  of  polarity  or  symmetry  of  a  whole 
organism  or  of  a  part.  These  geometrical  relations 
serve  primarily  to  express  in  a  general  way  the  fact  that 
spatial  order  of  certain  kinds  exists  in  the  organic  indi- 
vidual, but  the  orderly  sequence  of  events  in  time  is  also 
referable  to  a  greater  or  less  degree  to  the  geometrical 
scheme  of  the  individual  or  part.  In  development  the 
specialization  and  differentiation  make  their  appearance 
and  undergo  their  progressive  changes  in  a  more  or  less 
definite  sequence  with  respect  to  the  chief  axes.  In 
many  cases  the  original  geometrical  plan  of  the  indi- 
vidual undergoes  modification  or  gives  place  to  a  differ- 
ent plan.  Such  changes  are  sometimes  brought  about 
by  conditions  within  the  individual  and  sometimes 
occur  in  response  to  changes  in  external  conditions.  In 
general,  however,  it  may  be  said  that  in  any  given  kind 
of  individual  the  plan  is  always  the  same  or  undergoes  the 
same  changes  and  is  always  worked  out  in  essentially 
the  same  way  during  development,  provided  external 
conditions  are  the  same.  Under  altered  external  con- 
ditions departures  from  the  plan  may  occur,  and  indi- 
viduals result  which  differ  more  or  less  widely  from  the 


12  INDIVIDUALITY  IN  ORGANISMS 

usual  form  or  structure.  As  a  matter  of  fact,  no  two 
individuals  are  exactly  alike,  and  there  is  abundant 
reason  to  believe  that  this  is  so  because  no  two  individ- 
uals are  or  have  been  subjected  to  exactly  the  same 
conditions. 

From  whatever  standpoint  we  view  the  facts  we 
must  always  return  to  the  conclusion  that  the  unity  and 
order  so  characteristic  of  the  life  of  the  organic  individual 
are  in  some  way  or  other  an  expression  of  a  fundamental 
ordering  and  determining  capacity  of  some  sort  which 
makes  the  individual  what  it  is. 

REPRODUCTION   AND   INDIVIDUATION 

Reproduction,  the  formation  of  new  individuals 
from  parts  of  those  already  existing,  occurs  in  all  living 
forms,  and  the  question  of  the  origin  of  the  new  individual 
and  of  the  process  by  which  the  part  becomes  a  new  whole 
individual  is  perhaps  the  most  interesting  aspect  of  the 
whole  problem  of  individuality.  In  the  agamic  or 
asexual  forms  of  reproduction,  which  give  rise  to  new 
complete  organisms  in  the  plants  and  lower  animals,  we 
see  the  existing  individual  dividing  into  two  or  more, 
sometimes  in  a  very  regular  and  definite  manner,  some- 
times apparently  falling  apart,  as  it  were,  into  frag- 
ments or  single  cells;  or  it  gives  rise,  sometimes  in  a 
particular  region,  sometimes  in  regions  manifestly 
determined  by  chance  factors,  to  one  or  more  small 
outgrowths,  buds,  which  increase  in  size  and  become  new 
individuals,  and  these  in  some  cases  remain  organically 
connected  with  the  parent,  in  others  become  completely 
separated  and  independent.  In  many  cases  the  char- 
acter of  these  reproductive  processes  varies  with  the 


THE  PROBLEM 


13 


physiological  condition  of  the  individual  and  often  also 
with  external  conditions.  In  some  organisms,  for 
example,  many  kinds  of  reproductive  processes  occur, 
their  character  varying  with  internal  and  environmental 
conditions.  - 

Very  generally  it  is  possible  to  distinguish  more  or 
less  clearly  an  orderly  character  in  these  reproductive 
processes;  some  of  them  are  in  fact  orderly  to  a  high 
degree.  But  they  differ  so  widely  in  different  organisms 
that  attempts  to  discover  common  fundamental  factors 
underlying  the  various  forms  of  the  processes  have  not 
been  very  successful. 

In  addition  to  those  reproductive  processes  which 
give  rise  to  whole  new  organisms  there  are  also  those 
which  result  in  reduplication  or  more  or  less  complex 
parts.  The  repetition  of  radially  arranged  parts,  such, 
for  example,  as  tentacles  in  a  sea-anemone,  arms  in  a 
starfish,  a  whorl  of  leaves  or  the  parts  of  a  flower  in  a 
plant,  and  on  the  other  hand  the  succession  of  parts 
along  an  axis,  leaves  or  branches  along  the  stem  of  a 
plant,  or  the  segments  in  the  body  of  the  earthworm,  are 
all  reproductive  processes  and  involve  processes  of  indi- 
viduation. All  such  reproductive  processes  must  be 
included  in  any  attempt  at  a  theory  of  reproduction. 

And  finally  there  remains  the  process  of  sexual  or 
gametic  reproduction  in  which  the  union  of  two  cells, 
the  gametes  or  their  nuclei,  is  followed  by  a  series  of 
developmental  changes.  In  most  cases  of  gametic  re- 
production the  two  gametes  are  sexually  differentiated 
as  parts  of  two  different  individuals  or  in  different  organs 
of  the  same  individual  before  they  come  together. 
Moreover,   they  are  themselves  individuals,  and  their 


14  INDIVIDU.\I.ITY  IN  ORGANISMS 

union  results  in  a  new  individuation.  In  the  higher 
animals  this  form  of  reproduction  is,  with  very  rare 
exceptions,  the  only  process  which  gives  rise  to  organisms. 

Apparently  gametic  and  agamic  reproduction  are 
very  different  processes,  but  we  must  at  least  raise  the 
question  whether  they  are  similar  in  any  way,  or,  if 
they  are  different,  what  the  difference  may  signify. 

It  is  in  those  parts  of  pre-existing  individuals  which 
become  new  whole  individuals  that  the  process  of  indi- 
viduation goes  on  before  our  very  eyes,  and  it  is  there 
that  we  have  the  opportunity  to  determine  something  "of 
its  nature.  It  is  by  no  means  necessary  for  us  to  wait 
for  the  occurrence  of  reproduction  in  nature.  In  many 
of  the  simpler  organisms  we  can  bring  about  the  occur- 
rence of  reproduction  at  will,  simply  by  cutting  out 
pieces  of  the  body  and  so  isolating  them  from  their 
physiological  relations  with  other  parts.  Such  pieces 
may  become  new  organisms  or  parts  of  organisms  more 
or  less  like  those  from  which  they  were  taken.  These 
experimental  reproductions  constitute,  as  I  shall  show, 
invaluable  material  for  the  study  of  the  organic  indi- 
vidual and  of  the  process  of  individuation,  although  their 
value  for  this  purpose  has  not  heretofore  been  generally 
recognized. 

aiETABOLISM  AND  PROTOPLASM 

The  living  organism  consists  of  a  substance,  or  more 
properly  a  complex  mixture  of  substances,  in  which  the 
series  of  chemical  reactions  known  as  metabolism 
occurs.  The  fundamental  constituents  of  protoplasm 
occur  in  what  is  known  as  the  colloid  condition,  i.e.,  they 
do  not  form  a  true  molecular  solution,  but  exist  as  sus- 


THE  PROBLEM  15 

pended  particles  larger  than  molecules  in  the  fluid 
medium,  which  in  the  case  of  protoplasm  is  water. 
Living  protoplasm  may  range  in  its  physical  condition 
from  a  semi-fluid  to  a  stiff  jelly-like  substance  according 
to  the  aggregate  condition  of  its  particles.  This  mix- 
ture of  substances,  protoplasm,  is  the  visible  substratum 
of  the  living  form ,  and  in  it  the  changes  which  constitute 
hfe  occur.  Changes  in  its  aggregate  condition  and  in 
the  chemical  constitution  of  one  or  more  of  its  parts 
form  the  basis  of  specialization  and  differentiation  and 
so  of  structure  and  form,  and  the  energy  of  the  organism 
originates  from  certain  of  the  chemical  reactions  which 
occur  in  it. 

Metabolism  consists  in  a  complex  series  of  inter- 
related chemical  reactions  in  protoplasm.  On  the  one 
hand,  nutritive  substances  are  transformed  and  built 
up  into  protoplasm  or  into  other  substances  characteristic 
of  living  organisms,  and,  on  the  other  hand,  portions  of 
the  protoplasm  and  of  these  other  substances  are  broken 
down  and  oxidized,  setting  free  energy,  which  appears  in 
the  various  activities  of  life.  What  we  know  of  metab- 
olism indicates  that  the  oxidations  are  in  general  of 
fundamental  importance  in  the  whole  reaction  system. 
Apparently  life  cannot  continue  without  them,  and  the 
other  reactions  are  to  a  greater  or  less  extent  associated 
with  and  dependent  upon  them.  In  a  given  organism, 
under  given  external  conditions  the  rate  of  oxidation  is 
in  some  degree  a  measure  of  metabolic  activity  and  of 
life.  Objection  is  sometimes  made  to  the  term  "metab- 
olism" because  of  its  vagueness.  It  is  of  course  true 
that  we  do  not  know  all  the  various  reactions  and  their 
relations  to  each  other  and  to  other  conditions,  but  we 


i6  INDIVIDUALITY  IN  ORGANISMS 

do  know  that  for  a  given  organism  metabolism  is  in 
general  a  definite  and  characteristic  system  of  reactions 
subject  to  variation  with  change  in  conditions  but  never- 
theless maintaining  in  the  long  run  a  certain  rate  and 
character. 

In  general  terms,  protoplasm  is  the  foundation  of 
structure  and  form,  and  metabolism,  of  function,  in  the 
organism.  The  relation  between  structure  and  function 
has  been  the  subject  of  much  discussion.  For  some  the 
organism  possesses  a  certain  structural  organization 
which  arises  in  some  way  or  other  quite  independent  of 
function  and  w^hich  makes  function  possible,  just  as  a 
man-made  machine  possesses  a  certain  structure  which 
makes  its  function  possible.  Such  an  organism  must  be 
constructed  before  it  can  begin  to  function,  and  hy- 
potheses of  this  character  are  chiefly  concerned  with 
the  supposed  method  of  construction.  This  conception 
of  the  organism  ignores  the  fact  that  it  is  always  func- 
tioning while  it  is  alive:  life  is  function.  In  no  case  does 
the  organism  begin  to  function  only  after  its  construction 
is  completed;  it  always  functions  from  the  beginning; 
it  constructs  itself  by  functioning,  and  the  character  of  its 
functional  activity  changes  as  its  structural  develop- 
ment progresses.  Structure  and  function  are  mutually 
related.  Function  produces  structure  and  structure 
modifies  and  determines  the  character  of  function. 

Here  it  is  possible  to  refer  only  very  briefly  to  a  con- 
ception of  the  relation  between  structure  and  function 
which  I  have  discussed  more  at  length  elsewhere.' 
According  to  this  view  protoplasm  and  structure  rep- 
resent primarily  those  products  of  metabolism  which 

*  Child,  Senescence  and  Rejuvenescence,  1915,  pp.  26-31. 


THE  PROBLEM 


17 


are  relatively  stable  under  the  ordinary  physiological 
conditions  and  in  such  physical  condition  that  they  can- 
not escape  from  the  organism  without  change.  There- 
fore they  accumulate,  and  their  accumulation  constitutes 
growth,  and  their  differences  in  different  parts  constitute 
the  morphological  structure  of  the  organism.  The  less 
stable  products  appear  only  temporarily  or  not  at  all  as 
structural  features,  for  they  are  decomposed  and  elimi- 
nated. These  differences  in  stability  are  of  course  only 
relative  and  between  extremes  numerous  intermediate 
degrees  occur.  Moreover,  a  structure  which  is  stable 
under  certain  conditions  may,  under  altered  conditions, 
become  unstable  and  be  broken  down  and  replaced  by 
other  structures.  In  general,  structural  stability  in- 
creases both  during  the  development  of  the  individual 
and  the  course  of  evolution.  The  evolutionary  increase 
in  structural  stability  is  in  fact  what  makes  possible 
the  structural  permanency  and  complexity  of  the  higher 
as  compared  with  the  lower  organisms. 

If  the  organic  individual  is  a  physico-chemical  entity 
of  this  kind  the  foundation  of  its  unity  and  orderly 
character  must  be  present  somewhere  and  somehow  in 
this  metabolic-protoplasmic  system.  Definite  relations 
in  both  space  and  time  must  exist  among  the  reactions 
occurring  in  the  protoplasm,  and  the  problem  of  indi- 
viduality resolves  itself  into  the  problem  of  the  nature, 
origin,  and  maintenance  of  these  relations.  It  is  with 
the  problem  in  this  form  that  this  book  is  chiefly  con- 
cerned. 

TERMINOLOGY 

In  order  to  avoid  confustion  and  for  the  sake  of  con- 
venience and  brevity  it  is  necessary  to  fix  upon  and 


i8  INDIVIDUALITY  IN  ORGANISMS 

define  certain  terms  to  be  used.  The  individual  which 
forms  the  starting-point  of  a  developmental,  reproductive, 
or  life-history  I  shall  call  the  primary  individual.  This 
primary  individual  may  give  rise  by  reproduction  to 
secondary  individuals^  or,  by  the  individuation  of  certain 
organs  within  itself,  to  partial  or  organ-individuals. 
When  such  secondary,  partial,  or  organ-individuals 
continue  to  constitute  parts  of  the  unity  of  the  primary 
individual  it  is  the  dominant  individual  and  they  are 
subordinate  individuals.  The  segments  of  the  earth- 
worm body  or  the  leaves  of  a  plant  are  such  subordinate 
individuals.  When  the  primary  and  secondary  indi- 
viduals each  constitute  a  more  or  less  distinct  unity 
though  still  organically  connected  they  are  co-ordinate 
individuals.  In  many  trees  and  in  some  branching 
colonial  animals  various  branches  approach  or  attain  the 
condition  of  co-ordinate  individuals.  Between  strictly 
co-ordinate  and  the  extremes  of  dominant  and  sub- 
ordinate individuals  there  are  of  course  various  inter- 
mediate degrees.  A  common  or  general  individuality 
resulting  from  the  physiological  combination  of  a 
number  of  more  or  less  co-ordinate  individuals,  either 
similar  or  of  different  kinds,  is  a  composite  individual. 
Strictly  speaking,  all  organisms  except  perhaps  some  of 
the  simplest  unicellular  or  monoplastic  forms  are  to 
some  extent  composite  individuals  for  different  cells, 
and  even  different  parts  of  a  cell  may  possess  a  physio- 
logical unity  and  order  of  their  own,  but  ^nce  the 
following  chapters  are  chiefly  concerned  with  the  larger, 
more  general,  features  of  organic  individuality  rather 
than  wdth  its  more  minute  details,  the  term  will  be  used 
primarily  for  the  more  extreme  cases  in  which  a  number 


THE  PROBLEM 


19 


of  morphologically  and  physiologically  well-defined  and 
usually  multicellular  individuals  make  up  a  relatively 
persistent  composite  individual.  Most  plants  and  the 
so-called  colonial  animal  forms  are  good  examples. 
The  individuals  which  make  up  a  composite  individual 
are  constituent  individuals.  These  may  be  either  parts 
of  a  cell,  different  cells,  or  cell  groups  composing 
organs. 

As  regards  the  various  axes  of  the  axiate  individual, 
uniformity  of  designation  is  also  highly  desirable.  The 
polar,  longitudinal,  apico-basal,  or  antero-posterior 
axis,  as  it  is  variously  called,  represents  the  primary 
or  major  order  when  such  an  order  is  present  in  the  in- 
dividual. In  cases  where  the  axes  of  the  individual 
arise  de  novo  and  are  not  simply  carried  over  from  pre- 
existing individuals,  this  axis  is  apparently  the  first  to 
arise  and  other  axes  arise  in  relation  to  it.  It  is  often 
convenient,  therefore,  to  call  this  axis  the  major  axis  of 
the  individual  and  the  other  axes  minor  axes. 

With  reference  to  particular  axes,  we  are  accus- 
tomed to  distinguish  position  and  direction  according 
to  the  general  plan  of  the  individual,  the  relation  of 
certain  axes  to  others,  the  characteristic  position, 
behavior,  or  direction  of  movement  of  the  organism. 
The  following  terms  are  commonly  used  for  this  pur- 
pose: apical  and  basal,  distal  and  proximal,  anterior  and 
posterior,  peripheral  and  central,  median  and  lateral, 
dorsal  and  ventral,  besides  various  others  which  refer 
to  particular  regions,  such  as  cephalic  and  caudal,  oral 
and  aboral,  etc.  All  these  terms  are  useful  in  particular 
cases,  but  greater  uniformity  and  simplicity  are  desir- 
able for  purposes  of  general  consideration. 


20  INDIVIDUALITY  IN  ORGANISMS 

As  following  chapters  will  show,  there  is  reason  for 
believing  that  what  we  call  an  axis  in  the  organism 
represents  the  general  course  and  direction  of  a  gradient 
in  rate  of  metabolic  reactions,  the  rate  of  reaction  being 
highest  at  one  end,  or  in  a  certain  region,  and  decreasing 
from  this  point  in  the  direction  in  which  we  conceive 
the  axis  to  extend.  Moreover,  the  physiological  and 
structural  order  along  any  axis  is  definitely  related  to  this 
gradient.  If  all  organic  axes  are  fundamentally  meta- 
bolic gradients  we  may  call  the  region  of  highest  rate 
in  any  axis  the  apical  region  or  end,  the  region  of  lowest 
rate  the  basal  region  or  end,  while  other  intermediate 
regions  may  be  distinguished  as  more  or  less  apical  or 
basal,  and  opposite  directions  in  the  axis  as  respectively 
apical  and  basal  directions.  From  this  point  of  view 
apical  and  basal  regions  of  radially  symmetrical  whole 
organisms  are  merely  the  apical  and  basal  regions  of  the 
major  axis  of  such  organisms  and  so  the  most  conspicu- 
ous or  most  widely  separated  apical  and  basal  regions 
of  the  body,  but  not  fundamentally  different  in  their 
dynamic  significance  from  the  corresponding  regions  of 
other  axes.  In  the  following  pages  it  will  often  be  con- 
venient to  use  these  terms,  "apical"  and  "basal,"  in 
this  general  way  for  bilaterally  as  well  as  for  radially 
symmetrical  forms. 


CHAPTER  II 
THEORIES  OF  ORGANIC  INDIVIDUALITY 

Having  formulated  the  problem,  it  is  necessary  to 
in.quire  what  progress  has  already  been  made  toward  its 
solution.  The  first  section  of  this  chapter  is  a  very 
summary  consideration  of  this  question.  Since  the 
experimental  and  observational  data  upon  which  my 
own  conclusions  are  based  are  so  varied  and  their  rela- 
tions to  the  problem  in  many  cases  so  complex,  the 
inductive  method  of  procedure  is  impossible  within  the 
limits  of  the  present  book.  It  has  seemed  necessary, 
therefore,  to  state  my  conclusions  briefly  in  categorical 
form  as  a  working  hypothesis  before  attempting  to  review 
and  interpret  the  various  lines  of  evidence.  This  I 
have  attempted  to  do  in  the  second  section  of  the  chapter. 

THEORETICAL  REVIEW  AND   CRITIQUE 

The  organic  individual  has  very  often  been  compared 
to  a  human  society  or  state  with  orderly  division  of 
labor  and  correlation  among  its  component  parts. 
The  fundamental  feature  of  the  human  state,  that 
which  distinguishes  it  from  a  mere  aggregation  of 
human  beings  and  makes  it  an  individual,  is  some  kind 
and  degree  of  law  and  order,  of  co-ordination  and  control 
of  the  activities  of  its  constituent  units;  in  short,  some 
degree  and  kind  of  government.  If  the  organism  is  a 
cell-state  or  organ-state  some  degree  and  kind  of  govern- 
ment must  exist  in  it,  but  in  making  such  comparisons 
biologists   have    often   ignored    or    failed    to  recognize 


2J 


22  INDIVIDUALITY  IN  ORGANISMS 

the  importance  of  this  fundamental  point.  There  has 
been  much  discussion  of  "formative  substances"  and 
their  distribution  and  role,  and  the  magic  word  "organi- 
zation" has  served  as  the  all-sufficient  answer  to  many 
questions,  while  the  fundamental  problem  of  unity  and 
order  involved  in  the  origin  and  action  of  the  so-called 
formative  substances  and  in  the  nature  of  organization 
has  too  often  been  completely  neglected. 

Various  theories  of  the  organism,  which  may  be  called 
corpuscular  theories,  have  been  advanced  and  have  met 
with  more  or  less  general  acceptance.  Among  these 
Weismann's  germ-plasm  hypothesis  is  most  familiar  and 
has  played  the  most  important  role  in  biological  thought. 
These  theories  postulate  in  one  form  or  another  a  multi- 
tude of  specific  material  entities,  each  of  v/hich  represents 
in  some  way  some  characteristic  of  the  organism.  The 
organism  as  we  know  it  is  the  product  of  their  combined 
and  harmonious  activity.  Examination  of  these  theories 
shows  that  these  hypothetical  entities,  gemmules, 
determinants,  physiological  units,  pangenes,  specific 
accumulators,  or  whatever  we  prefer  to  call  them,  are 
themselves  endowed,  ex  hypothesi,  with  the  essential 
characteristics  of  individuals  and  that  the  organism  as 
a  whole  is  merely  a  composite  of  their  orderly  activities. 
Neither  the  problem  of  the  individuality  of  the  hypo- 
thetical units  nor  that  of  their  orderly  combination  and 
unification  in  the  organism  receives  any  adequate  con- 
sideration in  those  theories.  They  merely  translate 
the  problem  into  hypothetical  terms  which  are  beyond 
the  reach  of  scientific  method.  The  combination  of 
these  units  into  the  individual  is  assumed  to  occur  as  the 
facts  demand,  and  although  the  problem  of  the  control 


THEORIES  OF  INDIVIDUALITY 


23 


and  ordering  of  millions  of  such  units  through  all  the 
changes  involved  in  the  development  of  a  complex  organ- 
ism, say  the  human  being,  is  one  which  staggers  human 
intelligence,  it  is  practically  ignored.  Even  some  of 
our  present-day  speculations  which  attempt  to  assign 
actual  topographic  positions  in  the  chromosomes  to  par- 
ticular factors  in  heredity  ignore  completely  the  prob- 
lem of  the  ordering  and  control  of  these  factors  which  is 
involved  in  their  assumptions.  In  fact,  if  we  subject 
this  group  of  theories  to  logical  analysis  we  soon  reach 
the  point  where  it  is  necessary  to  assume  the  existence 
of  something  very  like  a  superhuman  intelligence  as  the 
underlying  principle  in  all  of  them.  They  leave  the 
essential  problem  unsolved,  but  their  implications  are 
anthropomorphic  and  teleological. 

Dualistic  or  ^'vitalistic"  theories  of  the  individual 
recognize  the  real  problem  more  or  less  clearly,  but 
assume  the  existence  of  a  non-mechanistic  ordering  and 
controlling  principle.  Before  the  development  of  the 
experimental  method  in  biology  the  doctrine  of  vital 
force  as  something  peculiar  to  the  organism  and  funda- 
mentally different  from  the  forces  acting  in  the  inorganic 
world  was  very  generally  accepted,  but  as  evidence  for 
the  validity  of  physico-chemical  laws  in  the  activities 
of  living  things  accumulated,  the  hypothesis  of  vital 
force  was  discarded  by  most  biologists.  Within  recent 
years,  however,  various  attempts  have  been  made  to 
show  the  inadequacy  of  mechanistic  conceptions  of  life. 
Driesch,  at  present  the  chief  exponent  of  this  line  of 
thought,  has  postulated  the  existence  of  a  controlling 
and  ordering  principle  which  he  calls  entelechy,  following 
Aristotle.      Entelechy  is  independent  of  and  superior 


24  INDIVIDU.\LITY  IN  ORGANISMS 

to  physico-chemical  laws,  and  controls  and  orders  the 
physico-chemical  factors  in  the  organism  to  a  definite 
end  or  purpose.  It  constructs  the  organism  as  a 
man  constructs  a  machine.  In  many  respects  it 
resembles  human  intelligence,  but  seems  to  be  far 
superior  to  it.  Other  neo-vitalistic  theories  are  more 
or  less  similar  in  their  general  conceptions,  but  differ 
in  detail. 

In  certain  respects  these  theories  constitute  a  real 
advance  over  the  corpuscular  theories,  for  they  recog- 
nize and  state  more  or  less  clearly,  instead  of  ignoring, 
the  essential  problem.  For  the  present,  however,  most 
of  us  find  little  intellectual  satisfaction  in  the  solu- 
tion which  they  offer,  and  they  are  either  frankly  specu- 
lative or  involve  unwarranted  or  premature  assumptions, 
and,  like  the  corpuscular  theories,  they  place  the  prob- 
lem beyond  the  bounds  of  science. 

Various  attempts  at  solution  or  progress  toward 
solution  of  the  problem  of  organic  individuality  have 
been  made  along  physico-chemical  lines.  The  evident 
unity  and  order,  the  individuality  of  the  inorganic 
crystal,  together  with  the  discovery  of  the  existence  of 
fluid  crystals,  have  led  to  comparisons  of  the  organism 
with  the  crystal  and  so  to  hypotheses  which  postulate 
an  essentially  crystalline  character  for  organic  unity 
and  order.  According  to  these  hypotheses  the  laws 
underlying  this  unity  and  order  are  essentially  those 
governing  the  aggregation  and  arrangement  of  mole- 
cules. The  construction  of  the  orderly  framework  of 
the  organism  is  the  expression  of  such  laws,  and  its 
activities  represent  the  chemical  changes  which  go  on 
in  this  framework. 


THEORIES  OF  INDIVIDUALITY  25 

These  hypotheses  are  open  to  various  objections. 
The  crystal  consists  primarily  of  hke  molecules  though 
under  certain  conditions  some  crystals  may  show  differ- 
ences in  constitution  at  the  two  poles  resulting  from  the 
presence  of  other  substances  besides  the  primary  con- 
stituent of  the  crystal.  The  organism,  on  the  other 
hand,  is  a  complex  of  many  different  kinds  of  molecules, 
some  of  which  are  undergoing  breakdown  and  being 
built  up  anew  during  life,  and,  moreover,  there  is  no 
optical  or  other  evidence  that  protoplasm  in  general  is 
fundamentally  crystalline  in  structure.  The  unity  of 
the  crystal  is  a  static  unity,  a  unity  of  form  and  arrange- 
ment, and  disappears  or  is  replaced  by  another  unity 
when  chemical  change  occurs,  while  the  unity  of  the 
organic  individual  is  a  dynamic  unity  dependent  pri- 
marily for  its  existence  on  chemical  activity  and  dis- 
appearing when  such  activity  ceases.  To  believe  that 
metabolism  results  from  structure  and  ''organization" 
as  the  activity  of  the  man-made  machine  results  from 
its  structure  is  to  ignore  the  fact  that  metabolism  is  the 
formative  agent  in  the  organism.  Undoubtedly  crystals 
or  crystalloid  individuals  are  present  in  at  least  many 
organisms,  but  their  individuality  is  quite  distinct  from 
that  of  the  organism. 

Some  biologists,  while  not  admitting  that  the  organ- 
ism is  fundamentally  crystalline,  assume  that  its  con- 
stituent molecules  possess  unknown  properties  which 
determine  its  unity  and  order.  These  hypotheses  are 
open  to  the  same  objections  as  the  crystal  hypotheses. 
All  such  hypotheses  in  fact  proceed  on  the  assumption 
that  a  certain  more  or  less  complex  "organization" 
is   necessary   as   a   starting-point;     the   machine   must 


26  INDIVIDUALITY  IN  ORGANISMS 

somehow  be  constructed  before  it  can  run.  Actually, 
however,  the  organism  runs  throughout  its  construction 
from  the  condition  of  amorphous  protoplasm  to  that 
of  a  complex  anatomical  system. 

Modern  investigation  of  the  chemistry  of  the  organ- 
ism has  demonstrated  that  the  chemical  correlations,  as 
they  are  commonly  called,  which  exist  between  its  parts 
are  most  various  and  complex  and  often  highly  specific 
in  character.  Certain  parts  produce  substances  which 
are  essential  to  the  normal  activity  or  structure  of  other 
parts,  and  the  statement  is  frequently  made  that  every 
organ  in  the  body  is  an  organ  of  chemical  correlation, 
which  means  merely  that  it  produces  something  which 
plays  a  role  in  making  other  parts  what  they  are. 

On  the  basis  of  these  facts  the  hypothesis  has  been 
advanced,  and  is  at  present  widely  current,  that  the 
unity  and  order  in  the  organism  consist  primarily  in  such 
chemical  correlations.  These  chemical  correlations  de- 
pend upon  the  production  and  transportation  within  the 
organism  of  more  or  less  specific  substances,  and  it  is 
evident  that  parts  more  or  less  specifically  different 
must  be  present  in  order  to  produce  such  substances. 
These  hypotheses  provide  no  solution  of  the  real  prob- 
lem of  individuality,  for  they  all  involve  the  assumption 
of  an  underlying  order  or  ''organization"  which  makes 
orderly  chemical  correlation  possible.  To  return  to  the 
analogy  between  the  organism  and  the  state,  exchange 
between  human  beings  arises  from  the  existence  of  differ- 
ent individuals  with  different  needs.  In  order  that  the 
exchange  may  be  orderly  and  specific  in  character  some 
degree  of  unity  and  order  must  exist  in  the  activities  of 
the  parties  to  the  exchange.     This  order  may   result 


THEORIES  OF  INDIVIDUALITY  27 

from  the  authority  of  one  person  and  its  transmission  to 
others,  or  from  that  of  consensus  of  opinion,  but  in  either 
case  it  is  not  the  act  of  exchange  nor  its  character  which 
determines  this  order  but  the  order  that  determines  the 
exchange  and  its  character.  The  orderly  union  of  hu- 
man beings  to  form  an  individuahty  which  shows  the 
most  various  degrees  of  individuation  from  the  family 
through  the  clan  and  tribe,  etc.,  to  the  highly  developed 
modern  state  is  based  primarily  on  authority  of  some 
kind  and  its  transmission,  not  upon  the  material  relation 
of  the  production  and  transportation  of  substances. 
When  this  union  of  men  exists,  no  matter  how  primitive 
its  character,  the  substances  which  it  receives  in  ex- 
change may  play  a  very  important  part  in  determining 
the  character  and  course  of  its  further  development.  If 
the  unity  of  the  organic  individual  is  in  any  way  com- 
parable with  that  of  these  composite  social  individuals, 
it  is  evident  that  it  must  originate  in  some  ordering  or 
controlling  factor  which  makes  possible  the  existence 
and  orderly  and  definite  arrangement  of  specific  parts. 
These  two  types  of  relation — authority  or  dominance 
of  some  sort  and  its  transmission  to  subordinate  parts 
and  the  production  and  transportation  of  substances — 
represent  the  two  kinds  of  relation  possible  between 
persons,  organs,  cells  or  parts  of  a  cell,  so  far  as  direct 
mechanical  relations  of  contact,  pressure,  or  tension 
are  not  concerned.  The  unity  of  the  social  individual 
evidently  depends  primarily  upon  the  transmissive 
rather  than  the  transportative  kind  of  relation.  If 
the  organic  individual  is  in  any  way  comparable  to  it 
we  might  reasonably  expect  to  find  the  same  thing 
true  there. 


28  INDIVIDUALITY  IN  ORGANISMS 

Various  biological  theories  have  concerned  them- 
selves primarily  with  that  particular  aspect  of  unity  and 
order  which  appears  in  the  geometrical  relations  of  parts. 
These  are  commonly  known  as  theories  of  polarity  and 
symmetry,  but  since  polarity  and  symmetry  are  funda- 
mental features  of  organic  individuality,  these  theories 
must  be  regarded  as  theories  of  the  organic  individual. 
It  is  unnecessary  to  discuss  these  theories  particularly, 
for  they  fall  into  the  same  groups  as  those  already  con- 
sidered, and  are  open  to  the  same  objections.  They 
either  assume  the  existence  of  some  kind  of  structural 
order  or  ''organization,"  physical  or  chemical,  or  some 
sort  of  pre-existent  plan  or  pre-established  harmony, 
or  they  ignore  or  fail  to  recognize  the  real  problem  and 
postulate  migrations  or  distributions  of  formative  sub- 
stances, differences  in  tension,  permeability,  or  other 
properties,  as  if  such  factors  could  behave  in  an  orderly 
and  constant  way  without  a  constant  underlying  order 
of  some  sort. 

Some  few  biologists  have  attempted  to  deny  the 
existence  of  individuality  in  the  sense  of  a  definite 
determining  and  controlling  unity  and  order.  The 
basis  of  such  denials  is  usually  the  fact  that  organisms 
behave  differently  under  different  external  conditions, 
while  the  more  important  fact  that  a  definite  unity  and 
order  exists  in  these  different  reactions  is  completely 
overlooked. 

This  brief  consideration  of  the  various  lines  of 
biological  thought  concerned  with  the  problem  of  the 
individual  is  sufficient  to  show  that  the  problem  is  by  no 
means  solved.  The  remainder  of  the  present  book  is  an 
attempt  to  make  some  progress  toward  a  solution  of  the 


THEORIES  OF  INDIVIDUALITY  29 

problem  along  somewhat  different  lines  from  those 
already  considered.  My  own  investigations  in  this 
field,  extending  over  some  fifteen  years,  together  with 
the  facts  already  at  hand,  as  I  see  them,  have  forced 
me  to  the  conclusion  that  the  organic  individual  is 
fundamentally  neither  a  structural  system,  whether 
physical  or  "vitalistic"  in  character,  nor  a  system  of 
chemical  reactions,  but  rather  a  system  of  relations 
between  a  physical  substratum  or  structure  and  chemical 
reactions.  These  relations,  I  believe,  constitute  the 
fundamental  problem  of  life,  so  far  as  it  is  a  biological 
problem,  and  as  one  aspect  of  it  the  problem  of  biological 
individuality.  This  is  the  point  of  view  which  underlies 
the  conception  of  the  individual  presented  in  the  follow- 
ing pages.  Since  the  relations  between  protoplasmic 
substratum  and  chemical  reactions,  whatever  their 
physical  or  chemical  character  in  particular  cases,  are 
essentially  dynamic,  I  have  called  it  a  dynamic  con- 
ception. 

A.  DYNAMIC  CONCEPTION  OF  THE  ORGANIC  INDIVIDUAL 

The  foundation  of  unity  and  order  in  the  organic 
individual  is  the  transmission  of  dynamic  change, 
'' stimulus,"  ''excitation,"  from  one  point  to  another  in 
the  protoplasm.  In  the  course  of  such  transmission  the 
transmitted  change  undergoes  a  decrement  in  intensity 
or  energy  so  that  finally  at  a  greater  or  less  distance 
from  its  point  of  origin  it  becomes  inappreciable  or 
ineffective.  In  the  simplest  case  such  a  transmitted 
change  originates  in  a  region  of  high  metabolic  rate,  and 
transmission  occurs  to  regions  of  lower  rate.  The  region 
of  high  metabolic  rate  results  in  the  final  analysis  from 


30 


INDIVIDUALITY  IN  ORGANISMS 


the  action  of  factors  external  to  the  mass  acted  upon, 
whether  part  of  a  cell  or  a  cell  mass.  A  simple  schematic 
consideration  will  serve  to  make  these  points  clear. 
Let  us  assume  a  spherical  mass  of  living  protoplasm 
(Fig.  i)  which  is  morphologically  and  physiologically 
homogeneous  except  as  regards  the  essential  features  of 
protoplasm  or  cells.     Such  a  mass,  whether  consisting 


a 


Fig.  I. — Diagram  illustrating  the  origin  of  a  single  axial  gradient  in 
protoplasm:  a,  the  point  of  action  of  the  external  factor. 


of  a  single  or  of  many  cells,  possesses  no  axis,  is  undiffer- 
entiated, and  is  not  a  physiological  individual.  Now  let 
us  suppose  some  external  factor  which  increases  meta- 
bolic rate,  a  ''stimulus,"  to  act  on  this  mass  in  the  region 
a  of  its  surface.  The  first  result  of  such  action  is  an 
increase  in  the  rate  of  metabolic  or  of  certain  metabolic 
reactions  in  the  region  a.  This  is  followed  by  a  spread- 
ing or  irradiation  of  a  dynamic  change,  either  over  the 
surface  of  the  mass  or  through  it  from  the  region  a. 


THEORIES  OF  INDIVIDUALITY 


31 


This  change  is  fundamentally  a  transmission,  not  a 
transportation,  for  it  consists  in  the  passage  of  a  certain 
energetic  change  and  not  in  the  bodily  transportation  of 
substance.'  Such  a  process  of  transmission  may  be  com- 
pared to  the  spreading  of  waves  in  a  pond  from  the 
point  where  a  stone  is  thrown  into  the  water,  although 
it  probably  does  not  always  or  necessarily  consist  of  a 
series  of  rhythmical  changes  like  the  water  waves. 

The  question  of  the  nature  of  the  transmitted  or  con- 
ducted excitation  has  been  the  subject  of  much  investi- 
gation and  discussion,  and  many  different  attempts  to 
answer  it  have  been  made.  Recent  investigation  indi- 
cates, however,  that  whatever  its  exact  nature,  it 
involves  an  increase  in  metabolic  activity.  It  seems 
in  fact  to  be  a  wave  of  increased  chemical  activity 
spreading  from  the  point  of  origin  much  as  a  wave  spreads 
in  a  pond.  The  question  of  the  relation  of  the  electrical 
and  chemical  changes  observed  in  the  transmission  of 
excitation  in  protoplasm  does  not  concern  us  here. 
The  fact  of  transmission  and  the  increase  in  metabolic 
activity  in  connection  with  it  are  the  important  points 
for  the  present  purpose. 

The  transmission  of  excitations  is  one  of  the  char- 
acteristic features  of  living  protoplasm,  and  undoubtedly 
occurs  to  a  greater  or  less  extent  in  all  protoplasm.  In 
its  simplest  form  it  is  perhaps  little  more  than  a  spread- 
ing or  irradiation  to  a  greater  or  less  distance  of  the 
change  produced  at  the  point  of  origin,  but  in  its  most 

^  It  should  perhaps  be  noted  that  from  the  standpoint  of  current 
physico-chemical  theory  transmission  itself  may  be  regarded  as 
molecular,  atomic,  ionic,  or  electronic  transportation.  Nevertheless, 
the  differences  between  such  transportation  and  the  transportation 
in  mass  of  substances  is  sufficient  to  warrant  the  distinction  made. 


32  INDIVIDUALITY  IN  ORGANISMS 

highly  specialized  form,  the  nerve  impulse,  it  probably 
differs  more  or  less  widely  from  the  initial  change. 

The  second  point  of  importance  in  connection  with 
such  transmission  is  the  existence  oi  a  decrement  in 
intensity  or  energy  of  the  change  in  the  course  of  its 
transmission.  Apparently  a  part  of  its  energy  is  used 
in  overcoming  a  resistance  or  inertia  or  in  producing 
other  changes  which  play  no  part  in  further  transmission. 
The  existence  of  this  decrement,  which  may  be  called 
the  transmission-decrement,  determines  that  at  a  greater 
or  less  distance  from  its  point  of  origin  the  transmitted 
change  becomes  inappreciable  or  ineffective,  and  trans- 
mission does  not  proceed  farther.  In  Fig.  i  the  intensity 
of  excitation  or  the  amount  of  increase  in  metabolic 
activity  is  indicated  diagrammatically  for  different 
distances  from  the  point  of  origin  in  a  by  the  bands  of 
different  width  concentric  at  a.  The  limit  of  effective- 
ness of  transmission  depends  on  the  intensity  or  energy 
of  the  original  change  produced  at  a  and,  secondly,  upon 
the  character  of  the  protoplasm.  The  higher  the  con- 
ductivity of  the  surrounding  protoplasm — in  other  words, 
the  less  its  resistance  or  the  greater  its  sensitiveness  to 
the  transmitted  change — the  greater  the  distance  to 
which  the  change  will  be  transmitted  before  becoming 
ineffective,  and  vice  versa.  In  the  existence  of  this 
transmission-decrement  the  resemblance  to  the  trans- 
mission of  waves  in  water  and  to  various  other  forms  of 
physical  transmission,  such  as  electrical  transmission,  is 
also  apparent.  A  decrement  in  velocity  of  transmission 
accompanies  decrement  in  intensity,  at  least  in  certain 
forms  of  transmission,  but  is  not  of  primary  importance 
in  the  present  consideration. 


THEORIES  OF  INDIVIDUALITY 


33 


If  the  external  factor  acts  only  momentarily  at  a,  the 
increase  in  rate  of  reaction  at  a  is  usually  only  momentary 
or  of  short  duration,  and  a  sooner  or  later  returns  to  or 
approaches  its  original  condition,  perhaps  in  some  cases 
with  a  gradually  disappearing  rhythm  of  increase  and 
decrease  in  rate.  The  transmitted  change  consists  in 
this  case  of  a  wave  or  a  series  of  successively  decreas- 
ing waves  of  change. 

It  is  probable  that  even  the  occurrence  and  passage 
of  such  momentary  changes  as  these  in  a  substratum  so 
sensitive  and  so  intimately  associated  with  the  reactions 
as  protoplasm  produce  changes  which  persist  for  a  longer 
or  shorter  time  after  the  metabolic  change  has  dis- 
appeared, but  such  changes  are  usually  slight  or  inap- 
preciable. If,  however,  the  external  factor  continues 
to  act  on  a  for  a  sufficiently  long  time,  or  if  it  acts 
intermittently  with  sufficient  and  not  too  great  fre- 
quency or  intensity,  it  produces  sooner  or  later  more  or 
less  permanent  changes  in  the  protoplasm,  which  are 
most  marked  in  the  region  a  and  decrease  with  the 
transmission-decrement.  The  exact  nature  of  these 
changes  is  not  certainly  known,  but  their  effect  is  to 
increase  the  reactive  capacity,  to  alter  the  protoplasm 
so  that  in  the  absence  of  external  stimuli,  or  with  a 
given  intensity  of  external  stimulus,  a  rate  or  intensity 
of  chemical  reaction  exists  higher  than  the  rate  under 
similar  conditions  before  the  change.  In  the  terms 
generally  employed,  the  irritability  of  the  protoplasm  is 
increased. 

Since  this  change  is  greatest  in  the  region  a,  Fig.  j.. 
where  the  excitation  is  greatest,  and  decreases  with 
increasing    distance    from    this    region,    the    result    of 


34  INDIVIDUALITY  IN  ORGANISMS 

continued  or  frequently  repeated  excitation  is  the  estab- 
lishment of  a  gradient  in  the  condition  of  the  protoplasm 
which  constitutes  a  more  or  less  permanent  material 
substratum  for  a  persistent  metabolic  gradient  inde- 
pendent of  the  local  external  stimulus.  In  short  the 
effect  of  the  local  action  of  an  external  factor  on 
protoplasm  may  sooner  or  later  result  in  the  estab- 
lishment of  a  metabolic  gradient,  or  the  material  basis 
for  such  a  gradient,  which  persists  for  a  longer  or  shorter 
time  after  the  external  factor  has  ceased  to  act.  As  a 
matter  of  fact,  such  gradients,  once  established,  often 
persist  throughout  the  life  of  the  individual.  These 
gradients  may  be  directly  visible  in  a  graded  structure  of 
the  protoplasm  as  well  as  in  differences  in  rate  of  reaction, 
or  they  may  appear  only  or  chiefly  in  the  differences  in 
rate,  according  to  the  nature  of  the  protoplasm.  There 
is  considerable  evidence  to  show  that  when  once  estab- 
lished to  a  certain  degree  they  tend  to  persist  and  even  to 
become  more  marked,  because  the  rate  and  extent  of 
further  changes  in  the  protoplasm  at  different  levels 
of  the  gradient  are  determined  by  the  differences  in  rate 
of  reaction  at  these  different  levels.  As  a  rapidly  flow- 
ing stream  quickly  removes  from  its  channel  obstacles 
which  a  slowly  flowing  stream  removes  only  slowly  or 
not  at  all,  so  the  changes  in  protoplasm  which  make  a 
higher  rate  of  reaction  possible  are  more  rapid  and  more 
extensive  with  a  high  than  with  a  low  rate  of  reaction. 
If  these  considerations  are  correct,  and  there  are, 
as  will  appear,  many  facts  to  support  them,  it  is  evi- 
dent that  a  persistent  metabolic  gradient  associated 
with  a  material  gradient  in  the  protoplasmic  sub- 
stratum may  arise  as  the  result  of  the  local  or  differential 


THEORIES  OF  INDIVIDUALITY  35 

action  of  an  external  factor  on  a  morphologically  and 
physiologically  homogeneous  living  mass. 

The  formation  of  metabolic  gradients  in  another  way 
is  possible,  at  least  in  single  cells.  If,  for  example,  inac- 
tive substances  of  different  weight  from  the  active  pro- 
toplasm are  present  in  the  cell,  and  if  the  position  of 
the  cell  with  respect  to  the  force  of  gravity  remains 
unchanged  for  a  sufhciently  long  time,  the  inactive 
substances  and  active  protoplasm  may  be  more  or  less 
definitely  localized  in  different  parts  of  the  cell  and  so  a 
metabolic  gradient  may  result.  Again,  it  is  conceivable 
that  continued  intake  of  nutrition  at  some  particular 
point  of  the  cell  surface  might  load  that  portion  of  the 
cell  with  inactive  reserve  substances  and  so  give  rise 
to  a  gradient.  To  what  extent  the  origin  of  metabolic 
gradients  is  due  to  such  factors  as  this  is  still  a  question. 
In  the  frog's  egg  gravity  undoubtedly  contributes  to  in- 
tensify the  existing  gradient  by  bringing  about  a  further 
separation  of  the  heavier  yolk  granules  and  the  lighter 
protoplasm,  but  it  is  not  responsible  for  the  origin  of 
the  gradient.  Unquestionably  the  primary  factor  in 
the  origin  of  these  persistent  metabolic-protoplasmic 
gradients  is  in  most  cases  at  least  a  reaction-gradient, 
and  the  persistent  or  permanent  gradient  in  the  proto- 
plasmic substratum  is  secondary. 

Such  metabolic  gradients  are,  I  believe,  the  simplest 
expression  of  physiological  unity  and  order  in  living 
protoplasm,  and  at  the  same  time  they  are  the  simplest 
and  primary  form  of  the  organic  axes  of  so-called 
polarity  and  symmetry  and  the  starting-point  of  the 
mysterious  ''organization."  They  are  factors  in  deter- 
mining the  direction  of  growth  and  differentiation  and 


36  INDIVIDUALITY  IN  ORGANISMS 

so  are  the  basis  of  the  geometrical  space  relations  and 
the  sequences  in  time  which  arise  during  the  develop- 
ment of  the  individual.  They  may  then  be  called 
axial  gradients.  The  region  of  highest  rate  in  such  a 
gradient  is  the  apical,  the  region  of  lowest  rate  the  basal 
region  of  the  axis  which  represents  the  general  direction 
and  course  of  the  gradient. 

Other  factors  besides  actual  rate  of  metabolic  reac- 
tions are  doubtless  concerned  in  the  formation  and 
establishment  of  these  gradients  in  protoplasm,  but 
these  are  associated  either  with  the  rate  of  reaction  or 
its  change,  or  with  the  character  of  the  protoplasm  in 
which  the  reaction  occurs  and  have  to  do  rather  with 
particular  cases  than  with  the  gradient  in  general.  The 
intensity  of  reaction,  for  example,  is  probably  such  a 
factor.  A  sudden  or  very  rapid  increase  in  rate  on 
excitation  is  probably  more  effective  in  producing  trans- 
mitted changes  than  a  gradual  increase,  and  it  is  also 
probable  that  in  protoplasm  with  a  high  reaction- 
intensity  excitations  are  transmitted  to  greater  distances 
than  where  the  reaction-intensity  is  low.  Excitation 
and  transmission  are  undoubtedly  also  correlated  with 
the  physiological  stability  and  physico-chemical  con- 
stitution of  the  protoplasm.  Such  factors  as  these 
may  play  a  part  in  determining  length,  slope,  or  other 
characteristics  of  the  gradient,  but  the  primary  factor 
in  its  production  appears  to  be  rate  of  reaction. 

In  a  metabolic  gradient  a  relation  of  dominance 
and  subordination  exists  between  the  level  of  highest 
and  the  levels  of  lower  metabolic  rate.  A  brief  con- 
sideration will  show  that  this  relation  is  a  simple  and 
necessary  result  of  the  differences  in  rate  of  reaction. 


THEORIES  OF  INDIVIDUALITY  37 

In  the  first  place,  the  apical  region  (a,  Fig.  i)  is  the  chief 
factor  in  determining  the  rate  of  reaction  at  other  levels, 
for  in  the  varying  conditions  of  a  natural  environment 
it  responds  more  rapidly  or  with  a  higher  rate  of  reaction 
than  other  levels  of  the  gradient  to  external  exciting 
conditions,  and  it  is  also  more  sensitive  and  may  react  to 
conditions  which  produce  no  reaction  at  lower  levels 
of  the  gradient.  With  every  such  increase  of  metabolic 
rate  in  response  to  external  exciting  factors  a  gradually 
decreasing  wave  of  change  spreads  from  this  region  of 
highest  rate,  as  in  the  original  excitation  which  gave 
rise  to  the  gradient,  though  the  intensity,  velocity,  and 
limit  of  effectiveness  of  the  transmitted  change  may  be 
much  greater  than  in  the  original  transmission. 

This  change  transmitted  from  the  apical  region  a 
plays  the  chief  part  in  determining  the  metabolic  con- 
dition at  other  levels,  because  a  is  the  region  of  highest 
metabolic  rate  and  the  changes  transmitted  from  it  are 
more  intense  than  those  from  other  levels  and  because  the 
establishment  of  the  protoplasmic  gradient  makes  con- 
duction in  this  direction  more  effective  than  in  any  other. 
Consequently  the  region  a  dominates  or  controls  other 
regions  within  a  certain  distance  and  to  a  greater  or 
less  degree  by  influencing,  through  the  changes  trans- 
mitted from  it,  their  metabolic  rate.  Dominance  or 
control  of  one  part  over  another  in  the  organism  is 
fundamentally,  I  beheve,  a  matter  of  difference  in 
metabolic  rate,  the  region  of  higher  rate  being  dominant. 

If,  after  such  a  gradient  is  established,  some  other 
region,  such  as  b,  Fig.  2,  undergoes  excitation  at  the 
same  time  as  a  and  by  an  external  factor  of  the  same 
intensity   as    that    acting   at   a,    the    response   will  be 


38  INDIVIDUALITY  IN  ORGANISMS 

less  rapid  and  less  intense  than  that  of  a,  and,  as 
indicated  in  Fig.  2,  the  transmitted  change  will  be 
weaker,  perhaps  less  rapid,  and  its  limit  of  effectiveness 
less  than  that  arising  from  a.  The  influence  of  the 
region  h  must  then  be  less  than  that  of  a  in  determining 
the  metabolic  rate  in  other  regions  and  a  remains  the 
dominant  region. 


a 


Fig.  2. — Diagram  illustrating  origin  of  major  and  minor  gradients  in 
a  simple  case:  a,  apical  region  of  major  gradient;  h,  apical  region  of 
minor  gradient. 


to^ 


If,  however,  the  region  h  or  any  other  region  is  suffi- 
ciently intensely  or  sufficiently  often  locally  excited 
independently  of  a,  a  persistent  gradient  may  arise  with 
relation  to  h  without  destroying  that  related  to  a.  In 
such  a  case  two  dominant  regions,  a  and  Z>,  exist,  but  a 
may  still  dominate  6  to  a  greater  or  less  extent  unless 
the  reactive  capacity  or  irritability  of  h  becomes  equal 


THEORIES  OF  INDIVIDUALITY 


39 


to  that  of  a.  Every  other  point  in  the  mass,  so  far  as  it 
is  within  the  Hmit  of  effectiveness  of  both  a  and  h,  will  be 
subordinate  to  both  to  a  greater  or  less  degree,  and  its 
metabolic  condition  will  be  the  resultant  of  its  position 
in  the  two  gradients.  Such  an  organism  possesses  not 
only  a  polar  axis  or  gradient,  but  an  axis  or  gradient  of 
symmetry  as  well,  and  in  the  same  way  other  gradients 
and  other  relations  of  dominance  and  subordination 
may  arise.  Obviously  it  is  possible  for  various  gradients 
to  exist  simultaneously  in  a  living  mass,  and  their  rela- 
tions may  be  very  different  in  different  cases,  as  are  the 
relations  between  the  different  axes  in  organisms.  Inter- 
ference between  different  gradients  in  opposite  or  nearly 
opposite  directions,  or  obliteration  of  one  gradient  by 
another  of  higher  rate  of  reaction,  undoubtedly  occurs,  as 
following  chapters  will  show,  but  their  relations  need 
not  be  considered  here. 

The  physiological  dominance  of  one  part  over  another 
is  certainly  not  a  constant,  unchanging  relation,  but 
depends  upon  the  metabolic  rate  in  the  dominant  apical 
region  and  the  conductivity  of  other  regions.  The  meta- 
bolic rate  in  the  dominant  region  is  also  not  constant, 
but  must  fluctuate  with  changes  in  external  conditions. 
With  a  slight  rise  in  metabolic  rate  in  the  dominant 
regions  its  influence  on  other  regions  is  slight  and  does 
not  extend  far,  but  when  the  increase  is  great  the  degree 
of  dominance  is  greater  and  extends  to  a  greater  distance. 
Certainly  in  the  primitive  individual  these  relations  must 
be  regarded  as  constantly  undergoing  change  in  degree 
and  extent,  though  under  the  usual  conditions  they  must 
also  show  a  general  average.  So  for  each  level  there  will 
be  a  general  average  of  the  effect  of  the  transmitted 


40  INDIVIDUALITY  IN  ORGANISMS 

change  upon  it,  and  the  gradient  will  be  further  intensi- 
fied by  the  fact  that  slight  transmitted  changes  do  not 
reach  the  more  remote  parts  at  all  while  they  do  affect 
the  parts  nearer  the  dominant  region.  It  is  this  general 
average  which  determines  the  more  conspicuous  and 
lasting  effects  at  different  levels. 

The  continued  existence  of  a  metabolic  gradient  of 
this  kind  undoubtedly  determines  an  increase  in  the 
conductivity  of  the  protoplasm  for  the  transmitted 
excitation.  Many  facts  indicate  that  within  certain 
limits  the  occurrence  and  repetition  of  transmission 
increase  the  conductivity,  and  in  all  animals  except  the 
simplest  a  nervous  system  which  possesses  a  very  high 
degree  of  conductivity  develops  in  relation  to  the  primary 
gradients.  In  most  organisms  there  is  therefore  an 
.extension  of  dominance  during  development,  the  trans- 
mitted changes  become  effective  through  a  greater  dis- 
tance, and  their  limit  of  effectiveness,  w^hich  of  course 
determines  the  range  of  dominance,  becomes  farther  and 
farther  removed  from  the  point  of  origin.  This  extension 
of  dominance,  however,  is  itself  limited  by  the  changes 
known  as  senescence,  which  become  evident  in  a  general 
decrease  in  reaction  rate.  These  relations  of  parts, 
dependent  in  the  final  analysis  on  differences  in  meta- 
bolic rate,  constitute,  as  I  believe,  the  foundation  of 
unity  and  order  in  the  organic  individual,  the  starting- 
point  of  physiological  individuation. 

If  this  conclusion  is  correct,  the  organic  individual, 
as  a  living  entity  possessing  some  degree  of  physiological 
— not  merely  physical — unity  and  order,  consists  in  its 
simplest  forms  of  one  or  more  gradients  in  part  of  a  cell, 
a  cell,  or  a  cell  mass  of  specific  physico-chemical  consti- 


THEORIES  OF  INDIVIDUALITY  41 

tution.  The  process  of  individuation  is  the  process  of 
establishment  of  the  gradient  or  gradients  as  a  more 
or  less  persistent  condition,  and  the  degree  of  individua- 
tion depends  upon  the  permanency  of  the  gradient,  the 
metabolic  rate  in  the  dominant  region,  the  conductivity 
of  the  protoplasm,  and  probably  on  other  factors  as  well. 

From  this  point  of  view  the  assumption  of  a 
mysterious,  self-determined  organization  in  the  proto- 
plasm, the  cell  or  the  cell  mass  as  the  basis  of  physio- 
logical individuality  becomes  entirely  unnecessary.  The 
origin  of  physiological  individuality  is  to  be  found,  not 
in  living  protoplasm  alone,  but  in  the  relations  between 
living  protoplasm  and  the  external  world.  In  view  of 
the  fact  that  the  organic  individual  after  its  formation 
is  far  from  independent  of  its  environment,  it  is  difficult 
to  see  why  we  should  assume  that  it  is  independent  and 
self-determining  in  its  origin. 

It  must  not  be  supposed,  however,  that  every  new 
individual  originates  in  the  manner  described  above. 
When  the  axial  gradient  is  once  established  in  a  cell  or 
an  organism,  it  may  simply  persist  through  the  process 
of  cell  division  or  other  forms  of  reproduction  so  that 
the  unity  and  order  of  the  new  individual  represent 
■simply  the  unity  and  order  of  the  parent  or  a  part  of  it. 
In  such  cases  the  basis  of  individuality  is  inherited  from 
the  parent.  In  nature  we  find  both  possibilities  realized : 
physiological  individuality  may  arise  de  novo  through  1 
the  relation  between  living  protoplasm  and  its  environ- 
ment, or  it  may  be  inherited  from  previously  existing 
individuals.  To  put  it  more  concretely,  an  axial  gradient 
cannot  arise  in  the  first  instance  independently  of  con- 
ditions external  to  the  mass  of  protoplasm  concerned. 


42  INDIVIDUALITY  IN  ORGANISMS 

but,   once   established,   it   may   persist   through   many 
generations. 

The  question  at  once  arises  whether  a  quantitative 
gradient,  such  as  has  been  described,  constitutes  an 
adequate  basis  for  the  physiological  specialization  and 
structural  differentiation  which  arise  in  relation  to  the 
axes  of  the  individual  and  in  the  higher  organisms 
become  very  complex.  Organs  showing  very  definite 
qualitative  differences  in  chemical  constitution  and 
metabolism  and  great  differences  in  functional  activity 
develop  in  the  organism.  Qualitative  specific  differ- 
ences of  some  sort  are  commonly  believed  to  be  necessary 
as  a  starting-point  for  such  complexity,  hence  the  usual 
theoretical  assumption  of  some  sort  of  underlying 
organization  as  the  basis  of  organic  individuality. 
Some  of  the  facts  bearing  upon  this  question  will  be 
considered  in  later  chapters;  here  attention  may  be 
called  to  three  points:  first,  it  is  a  familiar  fact  of 
chemistry  that  purely  quantitative  differences  may  bring 
about  the  formation  of  qualitatively  different  products 
from  the  same  reacting  substances,  and  in  a  complex 
physico-chemical  system,  such  as  living  protoplasm,  the 
possibilities  for  the  origin  of  qualitative  from  quantita- 
tive differences  is  very  much  greater  than  in  the  simple 
chemical  reaction  in  the  test  tube;  secondly,  it  is  by  no 
means  clear  what  is  quantitative  and  what  is  qualitative 
in  organic  structure  and  form,  or  in  metabolism,  for 
many  structural  difterences  which  are  ordinarily  con- 
sidered as  qualitative  prove  on  analysis  to  depend  on 
quantitative  differences  in  certain  constituents  of  the 
complex;  and,  thirdly,  morphological  differences  usually 
regarded  as  qualitative  can  unquestionably  be  produced 


THEORIES  OF  INDIVIDUALITY  43 

and  controlled  experimentally  by  metabolic  changes 
which  are  primarily  quantitative.  The  morphology  of 
the  channel  of  a  rapidly  flowing  stream  is  very  different 
from  that  of  a  stream  which  flows  slowly,  and  there  can 
be  little  doubt  that  in  the  organism  substances  which  are 
decomposed  and  transformed  or  eliminated  with  a  high 
rate  of  reaction  remain  and  accumulate  in  the  protoplasm 
and  may  form  characteristic  morphological  features  when 
the  rate  of  reaction  is  low. 

In  this  connection  the  question  must  be  raised 
whether  the  transmitted  change  is  always  of  the  same 
sort  and  produces  the  same  effect  in  a  protoplasm  of 
given  constitution.  It  is  impossible  at  present  to  give  a 
definite  answer  to  this  question,  but  there  seems  to  be 
no  positive  evidence  to  show  that  the  qualitative  char- 
acter of  the  effect  is  determined  by  the  character  of  the 
transmitted  change,  although  it  has  often  been  assumed 
that  this  is  the  case.  It  is  very  probable  that  the 
chemical  or  physico-chemical  character  of  the  trans- 
mitted change  differs  more  or  less  widely  in  plants  and 
animals,  and  in  embryonic  protoplasm  as  compared  with 
the  fully  developed  meduUated  nerve,  but  the  effect  in 
each  case  seems  to  be  primarily  excitatory  and  quanti- 
tative. It  seems  even  possible  that  in  passing  through 
.different  tissues  the  character  of  the  transmitted  change 
may  differ  more  or  less  according  to  the  constitution 
of  the  tissues,  but  its  effect  may  still  remain  essentially 
quantitative. 

If  it  should  be  demonstrated  that  the  same  proto- 
plasm may  transmit  different  kinds  of  excitations,  then 
of  course  different  processes  of  morphogenesis  and 
differentiation    might    be    determined    by    the    specific 


44  INDIVIDUALITY  IN  ORGANISMS 

character  of  the  transmitted  changes  affecting  different 
regions.  The  demonstration  of  such  relations  would 
of  course  compHcate  our  conception  of  the  course  of 
development,  but  would  not  necessarily  alter  our  views 
concerning  the  fundamental  principles  of  individuation. 
The  problem  of  the  nature  of  transmitted  changes  in 
protoplasm  has  been  the  subject  of  much  experiment 
and  discussion  and  is  still  by  no  means  solved,  but  our 
knowledge  concerning  them  is  sufficient  to  permit  us  to 
formulate  a  working  hypothesis  of  the  organic  individual 
in  terms  of  these  transmitted  changes  rather  than  in 
terms  of  transported  chemical  substances. 

As  soon  as  local  differences  in  chemical  constitution 
of  the  protoplasm  arise,  whether  they  result  from  differ- 
ences in  metabolic  rate  or  from  differences  in  character 
of  the  transmitted  change,  the  relations  commonly  called 
chemical  correlation,  consisting  in  the  production  and 
transportation  of  different  specific  substances,  begin  to 
play  a  part,  and  from  this  point  on  these  chemical  rela- 
tions are  factors  of  great  importance  in  determining  the 
character  of  the  different  parts,  until  in  the  adult  stage 
of  the  highest  forms,  man  and  the  other  mammals,  the 
complexity  of  chemical  correlation  is  bewildering,  as 
the  work  of  recent  years  on  hormones  and  internal 
secretions  has  clearly  demonstrated.  From  the  point  of 
view  developed  here  chemical  correlation  is,  however, 
a  secondary  factor,  for  the  underlying  order  which 
determines  the  orderly  character  of  chemical  correlation 
consists  in  the  quantitative  gradients  which  arise  in  the 
living  mass. 

Since  a  transmission-decrement  in  energy  or  in 
intensity  of  the  transmitted  change  exists,  the  change 


c 


THEORIES  OF  INDIVIDUALITY  45 

is  effective  only  within  a  certain  limit  of  distance  which 
we  may  call  its  range,  and  since  physiological  dominance 
depends  upon  the  transmitted  change  it  is  similarly 
limited  in  range.  If  physiological  individuation  depends 
upon  dominance  of  this  sort  associated  with  the  meta- 
bolic gradient  determined  by  transmitted  changes,  the 
range  of  dominance  must  determine  a  physiological 
limit  of  size,  which  the  individual  cannot  exceed  without 
the  physiological  isolation  of  some  part  from  the  domi- 
nance which  previously  determined  the  individuality. 
As  already  pointed  out,  the  range  of  the  transmitted 
change  and  so  the  range  of  dominance  varies  with  the  rate 
of  reaction  in  the  dominant  region  and  with  the  conduc- 
tivity of  the  protoplasm;  therefore  the  physiological  size 
limit  of  the  individual  must  vary  with  the  same  factors. 
Reproduction  in  its  simplest  asexual  forms  results 
from  the  physiological  isolation^  of  parts  of  the  indi- 
vidual body  in  consequence  of  their  coming  to  lie  beyond 
the  physiological  limit  of  size.  Such  physiological 
isolation  may  result  from:  first,  increase  in  size  of  the 
body  of  the  individual  by  continued  growth  until  some 
part  of  it  is  brought  beyond  the  range  of  dominance; 
secondly,  decrease  in  the  range  of  dominance  and  limit 
of  size  by  decrease  in  the  rate  of  reaction  in  the  dominant 
region;  thirdly,  decrease  in  the  conductivity  of  the  proto- 
plasm for  the  transmitted  changes;  fourthly,  the  direct 
local  action  of  some  external  factor  on  a  subordinate 
part,  increasing  its  rate  of  reaction  to  a  sufficient  de- 
gree to  make  it  more  or  less  independent  of  or  insus- 
ceptible to  the  effects  transmitted  from  the  dominant 

^  Child,  "Die  physiologische  Isolation  von  Teilen  des  Organismus," 
Vortrdge  und  Aufsatze  iiber  Entmckeliingsmechanik,  H,  XI,  191 1. 


46  INDIVIDUALITY  IN  ORGANISMS 

region.  This  change  I  have  called  decrease  in  receptiv- 
ity of  the  subordinate  part  for  the  transmitted  change. 
The  effect  of  physiological  isolation  of  a  part  is 
essentially  the  same  as  that  of  physical  isolation.  In 
the  lower  organisms  where  its  physiological  and  morpho- 
logical characteristics  as  a  part  are  less  stable  than  in 
the  higher  forms  and  it  is  able  to  respond  to  the  altered 
conditions  accompanying  physiological  isolation,  it 
loses  more  or  less  completely  its  character  as  a  part 
because  the  conditions  which  determined  and  maintained 
its  specialization  no  longer  act.  Consequently  it  under- 
goes dedifferentiation  to  a  greater  or  less  degree  and  so 
approaches  or  returns  to  the  undifferentiated  or  embry- 
onic condition,  and  is  then  capable,  if  differences  in  meta- 
bolic rate  in  the  direction  of  the  original  gradient  or 
gradients  still  exist  in  it,  or  if  conditions  determine  the 
origin  of  new  gradients  in  it,  of  development  into  a 
new  individual.  I  have  shown  that  development  and 
differentiation  are  in  general  accompanied  by  a  decrease 
in  metabolic  rate  which  constitutes  physiological  senes- 
cence and  that  the  dedifferentiation  of  isolated  parts 
brings  about  rejuvenescence  varying  in  degree  with  the 
degree  of  dedifferentiation.^  New  individuals  formed 
from  physiologically  or  physically  isolated  parts  of  pre- 
existing individuals  may  therefore  be  physiologically 
younger  than  the  individuals  from  which  they  arose  and 
so  be  capable  of  repeating  the  developmental  history' 
and  process  of  senescence.  Asexual  reproduction  in 
general  results  from  such  physiological  isolation  of 
parts  and  their  dedifferentiation  and  redifferentiation 
into  individuals.     In  the  higher  animals  physiological 

'Child,  Senescence  and  Rejuvenescence,  1915;    particularly  chaps,  ii, 
iv,  V,  vi,  viii,  x,  xv. 


THEORIES  OF  INDIVIDUALITY  47 

isolation  of  parts  probably  does  not  occur  except  occa- 
sionally in  embryonic  stages,  for  with  the  evolution  and 
development  of  the  nervous  system  in  the  individual  the 
transmission-decrement  decreases  and  the  effective  range 
of  transmission  therefore  increases  until  in  the  nerves 
of  mammals  the  transmission-decrement  is  inappreciable 
under  natural  conditions  in  the  lengths  of  nerve  fiber 
available  for  experiment.  In  these  forms  the  physio- 
logical limit  of  size  of  the  individual  determined  by  the 
range  of  dominance  is  very  great  and  is  never  attained 
by  the  individual  because  growth  is  limited  by  the 
progress  of  differentiation  in  the  course  of  development. 
In  such  organisms,  then,  physiological  isolation  does  not 
occur  except  occasionally  in  embryonic  stages  before  the 
nervous  system  has  developed  or  under  special  condi- 
tions which  limit  the  range  of  dominance  or  decrease 
the  receptivity  of  subordinate  parts. 

Moreover,  in  the  higher  animals  the  degree  and 
stability  of  specialization  of  parts  of  the  body  is  so  great 
that  in  most  cases  they  do  not  respond  to  physiological 
or  physical  isolation  by  reproduction,  but  either  die  or 
remain  largely  unchanged.  For  these  reasons  asexual 
reproduction  among  the  higher  animals  is  rare  and  is 
limited  to  early  developmental  stages.  Sexual  or 
gametic  reproduction  which  results  from  the  union  of  the 
two  gametes  or  sex  cells,  which  are  usually  specialized 
and  differentiated  as  egg  and  sperm,  is  somewhat  more 
complex  than  asexual  reproduction,  but  I  have  already 
endeavored  to  show  that  there  is  a  fundamental  physio- 
logical similarity  in  the  two  processes,'  and  I  shall  con- 
sider the  question  briefly  in  a  later  chapter. 

^  Ibid.,  chaps,  vi,  x,  xiii,  xiv,  xv. 


48  INDIVIDUALITY  IN  ORGANISMS 

This,  then,  is  in  brief  the  dynamic  conception  of  the 
organic  individual  which  has  grown  out  of  years  of 
experimental  investigation,  observation,  and  analysis  of 
facts  already  at  hand.  Its  distinctive  feature  is  the 
interpretation  of  physiological  unity  and  order  in  terms 
of  differences  in  rate  of  reaction  and  of  transmitted 
changes,  instead  of  in  terms  of  a  hypothetical  organiza- 
tion and  of  transportation  of  chemical  substances.  Ac- 
cording to  this  conception  the  central  nervous  system  in 
its  relation  to  other  parts  is  merely  the  final  expression  of 
relation  which  is  the  foundation  and  starting-point  of 
organic  individuation.  This  conception  provides  a 
working  hypothesis  based  on  a  great  variety  of  evidence 
and  readily  accessible  to  experimental  and  analytic 
investigation,  and  while  it  is  manifestly  far  from  being 
a  complete  solution  of  the  problem  of  organic  individu- 
ality, I  believe  that  it  throws  some  light  on  various 
characteristics  of  the  organism  the  nature  and  sig- 
nificance of  which  have  heretofore  remained  obscure. 

It  is  perhaps  necessary  to  point  out  that  this  dynamic 
individuality  is  not  the  only  kind  of  individuality  which 
exists  in  the  organic  world.  Physical  individuals  of 
crystalline  or  crystalloid  character,  and  perhaps  physico- 
chemical  individuals  of  other  sorts  exist  in  organisms. 
It  is  not  with  these,  however,  that  we  are  concerned,  but 
with  that  sort  of  individuality  which  is  distinctive  of 
the  living  organism,  which  determines  harmonious 
development  and  functional  unity  throughout  the  con- 
tinuous dynamic  change  which  constitutes  life.  Where 
this  organic  individuality  makes  its  first  appearance  it 
is  impossible  to  say.  The  cell  or  protoplast  in  its 
simplest  terms  usually  shows  some  degree  of  such  indi- 


THEORIES  OF  INDIVIDUALITY  49 

viduation,  but  it  is  probable  that  some  real  or  apparent 
individuations  which  arise  temporarily  or  are  persistent 
in  the  cell  approach  more  nearly  the  inorganic  than 
the  organic  kind.  Nevertheless,  wherever  a  region  of 
high  metabohc  rate  arises  in  protoplasm,  there  some 
degree  of  organic  individuation  arises,  at  least  for  the 
time  being,  provided  relations  already  existing  do  not 
interfere  with  or  inhibit  the  establishment  of  a  metabolic 
gradient. 

According  to  the  dynamic  conception  organic  indi- 
viduality results  in  the  final  analysis  from  the  relations 
between  living  protoplasm  and  the  world  external  to  it. 
If  we  accept  this  view  we  should  expect  to  find  morpho- 
logical structure  and  differentiation  making  their  first 
appearance  in  the  superficial  regions  of  the  protoplasmic 
mass.  These  are  in  more  direct  relation  with  the 
external  world  and  therefore  more  irritable  and  with 
the  establishment  of  a  region  of  high  metabolic  rate  a 
metabolic  gradient  must  arise  much  more  rapidly  in  the 
superficial  than  in  other  regions.  The  facts  agree  well 
with  this  view,  for  the  first  indications  of  individuation 
in  the  organism  are  very  generally  superficial  and  in 
many  of  the  simpler  forms,  such  as  the  infusoria  among 
animals,  orderly  morphological  differentiation  is  always 
limited  to  the  superficial  regions.  The  nervous  system 
i§  also  superficial  in  origin.  In  the  plant  cells  also  the 
superficial  portions  of  the  cytoplasm  generally  show  a 
higher  degree  of  stabihty  than  other  regions  and  are 
apparently  chiefly  concerned  in  whatever  morphological 
protoplasmic  differentiation  occurs.  If  organic  indi- 
viduality is  self-determined  there  is  no  apparent  reason 
for  its  appearance  as  a  superficial  phenomenon. 


CHAPTER  III 
METABOLIC  GRADIENTS  IN  ORGANISMS 

If  metabolic  gradients  are  of  such  fundamental  impor- 
tance in  the  organic  individual  it  should  be  possible  to 
discover  various  proofs  or  indications  of  their  existence. 
This  chapter  is  a  survey  of  some  of  the  experimental 
and  observational  evidence  for  the  existence  of  metabolic 
gradients. 

SUSCEPTIBILITY   GRADIENTS   IN   ANIMALS   AND 

PLANTS 

The  resistance  or  susceptibility  of  living  protoplasm 
to  various  poisons  can  be  used,  with  certain  precautions 
and  within  certain  limits,  as  an  index  of  its  metabolic 
condition.  This  method,  which  may  be  called  the  sus- 
ceptibility method,  makes  it  possible,  particularly  in 
early  stages  of  development  and  in  small,  simple  animals, 
to  compare  the  susceptibilities  and  so  to  obtain  a  general 
idea  of  the  differences  in  metabolic  activity  of  differ- 
ent regions  of  the  body  of  a  single  organism.  Many 
different  substances  may  be  used  as  reagents  for  deter- 
mining susceptibility,  such,  for  example,  as  the  alcohols, 
ethers,  and  other  narcotics,  and  acids  and  alkalies. 
Various  products  of  metabolism,  among  them  carbon 
dioxide,  and  certain  conditions,  such  as  lack  of  oxygen, 
serve  the  same  purpose.  But  the  cyanides,  which  are 
powerful  poisons,  are  in  many  respects  the  most  satis- 
factory reagents,  and  they  have  been  used  in  most  of 
my  experiments. 

so 


METABOLIC  GRADIENTS  51 

The  relation  between  metabolic  activity  and  sus- 
ceptibility to  these  substances  is  primarily  quantitative, 
the  degree  of  susceptibihty  depending  upon  the  rate  or 
intensity  of  metaboKsm  or  of  certain  fundamental 
metabolic  reactions.  In  aqueous  concentrations  of  a 
given  reagent  which  kill  within  a  few  hours,  the  sus- 
ceptibihty varies  directly  with  the  general  metabohc 
rate;  the  higher  the  rate  of  metabohc  activity,  the 
sooner  does  death  occur.  In  very  low  concentrations, 
however,  to  which  the  organism  is  able  to  acclimate  or 
accustom  itself  to  some  extent,  we  find  the  relation 
reversed.  The  higher  the  metabolic  rate,  the  greater  the 
degree  of  acclimation  and  therefore  the  less  the  sus- 
ceptibility and  the  later  the  occurrence  of  death.  These 
two  methods  of  comparing  susceptibilities  I  have  called 
the  direct  and  the  acclimation  method. 

The  question  how  these  various  substances  act  upon 
the  living  organism,  whether  they  enter  directly  into  the 
chemical  reactions  or  whether  they  change  the  physical 
condition  of  the  protoplasm  or  certain  of  its  constituents 
in  such  a  way  that  the  reactions  cannot  continue,  has 
long  been  and  is  still  the  subject  of  discussion,  but  cannot 
be  considered  here.  Whatever  the  nature  of  their 
action,  there  can  be  no  doubt  concerning  the  general 
relation  between  susceptibility  to  them  and  metabolic 
condition,  although  under  certain  conditions  the  rela- 
tion may  be  masked  or  altered  by  certain  incidental 
factors. 

For  the  direct  form  of  the  method,  which  is  the 
simplest  and  most  widely  applicable,  the  procedure  con- 
sists in  the  immersion  of-  the  animals  to  be  examined, 
either  singly  or  in  lots,  in  a  concentration  of  cyanide 


52  INDIVIDUALITY  IN  ORGANISMS 

or  other  reagent  used,  which  has  been  previously  deter- 
mined as  a  concentration  which  will  kill  the  animals  in 
the  course  of  a  few  hours  under  the  given  conditions 
of  temperature,  etc.  In  many  of  the  lower  animals 
death  is  followed  at  once  or  in  a  few  moments  by  a 
visible  disintegration  and  complete  loss  of  structure 
and  form  of  the  part  concerned,  and  in  such  cases  the 
progress  of  death  can  be  directly  observed.  In  other 
cases  other  means  of  determining  the  death-point  may 
be  employed  or  the  animals  may  be  removed  from  the 
reagent  at  definite  intervals  and  the  progress  of  death, 
and  so  the  susceptibility,  determined  by  observing 
whether  and  to  what  extent  recovery  occurs  in  each 
case.  When  the  method  is  used  in  this  way  regions  of 
high  metabolic  rate  die  earlier  than  those  of  low  rate. 

In  the  indirect  or  acclimation  form  of  the  method  we 
find  that  the  degree  of  acclimation  varies  with  meta- 
bolic rate.  With  this  form  of  the  method  regions  of 
high  metabolic  rate  are  least  susceptible  in  the  long  run 
because  they  become  acclimated  more  readily,  while 
regions  of  lower  metabolic  rate  undergo  less  acclimation 
and  so  are  inhibited  to  a  greater  degree  and  may  even 
die.  The  susceptibility  gradients  observed  with  these 
two  modifications  of  the  method  are  themselves  opposite 
in  direction,  but  are  different  expressions  of  the  same 
metabolic  gradient.^ 

Several  species  of  the  fiatworm  Planaria  constituted 
the  material  for  my  first  observations  on  susceptibility 
gradients.     The  results  obtained  were  so  definite  and 

^  For  more  extended  discussions  of  this  method  see  Child,  Senes- 
cence and  Rejuvenescence,  1915,  chap,  iii;  also  "Studies  on  the  Dynamics 
of  Morphogenesis  and  Inheritance  in  Experimental  Reproduction,  V," 
Jour,  of  Exper.  ZooL,  XIV,  1913. 


METABOLIC  GRADIENTS  53 

striking  in  character  that  the  desirability  of  comparative 
study  of  different  forms  at  once  became  evident.  Up 
to  the  present  time  some  fifty  species  of  animals  from 
various  groups  have  been  examined  by  means  of  the 
susceptibility  method,  either  in  the  adult  or  embryonic 
stages  or  in  both,  in  the  attempt  to  determine  to  what 
extent  regional  differences  or  gradients  in  metabolic 
condition  with  respect  to  the  axial  or  any  other  directions 
in  the  body  are  characteristic  features  of  the  animal 
organism.' 

In  each  form  examined  a  more  or  less  distinct  and 
regular  gradient  in  susceptibility  has  been  observed  in 
the  direction  of  the  major  axis  of  the  body  and  in  many 
cases  gradients  in  the  direction  of  the  minor  axes  and  of 
the  axes  of  various  organs  and  parts  as  well.^ 

^  The  forma  examined  include  twelve  species  of  ciliate  infusoria 
among  the  protozoa,  the  post-embryonic  or  adult  stages  of  the  fresh- 
water hydra,  and  three  species  of  hydroids  among  coelentrates;  one 
ctenophore,  eleven  species  of  turbellaria,  and  certain  larval  stages  of  one 
trematode  among  the  flatworms.  Dr.  L.  H.  Hyman,  workmg  under  my 
direction,  has  examined  in  the  same  way  nine  species  of  oligochete  anne- 
lids and  one  polychete.  Susceptibility  studies  have  been  made  upon  the 
eggs  and  embryonic  or  larval  stages  of  the  following  forms:  starfish, 
sea-urchin,  the  polychete  annelids  Nereis,  Chaetopterus,  Arenicola, 
Hydroides  among  the  invertebrates,  and  two  species  of  fishes  and  the 
salamander  and  frog  among  the  vertebrates. 

2  The  data  concerning  susceptibility  gradients,  so  far  as  they  have 
been  pubhshed,  will  be  found  in  the  following  papers:  Child,  "Studies 
on  the  Dynamics  of  Morphogenesis  and  Inheritance  in  Experimental 
Reproduction,  I-V,  VII,  VHI,"  Jour,  of  Exper.  ZooL,  X,  XI,  XIII,  XIV, 
XVI,  XVII,  1911-14;  "Studies,  etc.,  VI,"  Archh  fur  Entwickelirngs- 
mechanik,  XXXVII,  1913;  "Certain  Dynamic  Factors  in  Experimental 
Reproduction  and  Their  Significance  for  the  Problems  of  Reproduction 
and  Development,"  Archiv  fiir  Entwickelungsmechanik,  XXXV,  1913; 
"Susceptibility  Gradients  in  Animals,"  Science,  XXXIX,  No.  993,  1914; 
"The  Axial  Gradient  in  Ciliate  Infusoria,"  Biol.  Bull.,  XXVI,  1914; 
"Axial  Gradients  in  the  Early  Development  of  the  Starfish,"  Amer. 
Jour,  of  Physiol.,  XXXVII,  1915. 


54  INDIVIDUALITY  IN  ORGANISMS 

In  organisms  or  parts  with  a  radial  structure  gradients 
in  susceptibility  may  commonly  appear  in  the  direction 
of  the  radial  axis,  and  in  those  animals  and  developmental 
stages  where  the  outer  body  surface  consists  of  active 
living  cells  and  is  not  covered  by  a  heavy  cuticle  or 
exoskeleton  a  susceptibility  gradient  from  the  surface 
inward  has  been  frequently  observed. 

In  the  simpler  multicellular  animals  and  in  those 
unicellular  organisms  which  possess  definite  permanent 
axes,  the  susceptibihty  gradients  along  the  main  body 
axes  often  persist  from  the  beginning  of  development 
throughout  Ufe  without  essential  change,  but  in  many 
cases  they  undergo  various  changes  during  the  course 
of  development:  they  may  disappear  and  new  gradients 
arise  with  advancing  differentiation  and  the  appearance 
of  new  organs,  or  they  may  undergo  reversal  in  direction 
in  some  or  most  of  the  tissues  of  the  body.  In  all  cases, 
however,  so  far  as  observed,  such  changes  occur  in  a 
definite  and  orderly  way,  so  that  the  relation  between  the 
original  and  the  final  condition  is  essentially  constant 
and  characteristic  for  a  given  species.  In  spite  of  the 
developmental  alterations,  it  is  true,  as  far  as  observa- 
tions go  at  present,  that  for  each  of  the  main  axes  of  the 
body  a  defuiite  susceptibility  gradient  exists,  at  least 
during  the  earlier  stages  following  the  appearance  of  the 
axis,  and  a  definite  relation  exists  between  the  direction 
of  the  gradient  from  high  to  low  susceptibility  along  a 
given  axis  and  the  course  of  development  and  differen- 
tiation and  the  functional  correlation  of  different  parts 
with  reference  to  the  same  axis. 

The  following  figures  will  serve  to  show  something 
of  the  definiteness  of  the  gradient  along  the  apico-basal 


METABOLIC  GRADIENTS 


55 


axis  in  single  cells.  Figs.  3-7  show  the  course  of  death 
and  disintegration  along  the  axis  in  Stentor  coendeiis, 
one  of  the  common  infusoria.  Fig.  3  represents  the 
normal    animal    in    extended    condition,    showing    the 


^\l\(f.^n\"^'  (L.,.,\^^ 


^Wi/i 


^ 


Figs.  3-7. — Axial  susceptibility  gradient  of  Stentor  in  cyanide: 
Fig.  3,  intact  animal;  Fig.  4,  beginning  of  disintegration;  Figs.  5-7, 
successive  stages  of  disintegration. 


flattened  peristome  at  the  free  apical  end  with  its  spiral 
of  large  cilia,  the  shorter  cilia  over  other  parts,  the  longi- 
tudinal striations  or  fibrillae,  and  the  elongated  basal 
region  with  organ  of  attachment . 


56  INDIVIDUALITY  IN  ORGANISMS 

In  cyanide  the  body  undergoes  some  contraction, 
death  begins  at  the  apical  end  (Fig.  4)  and  is  accom- 
panied by  the  instantaneous  loss  of  all  movement  and 
disintegration  of  structure  in  the  part  concerned,  and  the 
protoplasm  swells  and  spreads  out  in  the  water,  as  indi- 
cated by  the  dotted  outline  in  Fig.  4.  Other  parts 
remain  intact  and  the  cilia  continue  to  vibrate.  From 
the  apical  region  death  and  disintegration  proceed  along 
the  body  as  shown  in  Figs.  5-7,  the  line  of  demarcation 
between  the  dead  and  disintegrated  and  the  living 
portions  remaining  distinct  at  all  times  until  the  progress 
of  death  ends  at  the  basal  end  of  the  body.  The  rate 
of  progress  of  death  over  the  whole  body  may  vary  from 
a  few  seconds  to  five  or  ten  minutes,  according  to  con- 
centration of  cyanide  used,  temperature,  and  other  con- 
ditions. Deviations  from  this  course  are  very  rare  and 
are  probably  the  result  of  local  stimulations  of  one  part 
or  another  of  the  body. 

In  Fig.  8  the  beginning  of  death  and  disintegration 
in  the  unfertilized  starfish  egg  is  shown.  The  region  of 
the  egg  where  disintegration  begins  is  that  region  where 
the  nucleus  lies  nearest  the  surface.  When  the  egg 
develops  this  region  gives  rise  to  the  apical  end  of  the 
embryo  and  larva.  From  this  region  disintegration 
proceeds  through  the  egg  along  the  axis  determined  by 
the  eccentric  position  of  the  nucleus  (Fig.  9),  and  this 
axis  corresponds  with  the  major  axis  of  the  embryo  and 
larva.  The  same  susceptibility  gradient  also  appears 
in  embryonic  and  early  larval  stages.  In  these  cases  the 
death  gradient  does  not  indicate  the  presence  of  more 
than  one  axis.  In  many  forms  other  axes  are  also 
indicated  by  the  course  of  death.     In  the  embryo  of  the 


METABOLIC  GRADIENTS 


57 


frog,  for  example,  which  is  bilaterally  symmetrical  and 
in  which  three  axes,  the  major  or  longitudinal  axis  and  the 
minor  transverse  and  dorso-ventral  axes,  are  distinguish- 
able in  the  arrangement  of  parts,  disintegration  begins 
first  of  all  at  the  anterior  end  and  proceeds  posteriorly, 
and  at  any  level  of  the  body  it  begins  in  the  median 
dorsal  region  and  proceeds  laterally  and  ventrally. 

The  susceptibiHty  gradients  in  particular  organs  or 
parts  of  the  body  also  show  a  relation  to  the  axes  of  these 


\ 
J 


t  ; . 


^•.         -■.    ■    '■> 


Figs.  8,  9. — Axial  susceptibility  gradient  of  starfish  egg  in  cyanide 

parts.  In  the  elongated  tentacles  of  hydra  and  various 
sea-anemones,  for  example,  death  begins  at  the  tip  and 
proceeds  toward  the  base,  and  in  nerves,  so  far  as  exam- 
ined, a  susceptibility  gradient  exists  and  death  proceeds 
in  the  direction  of  conduction. 

Many  other  examples  might  be  cited  to  show  the 
relation  between  the  progress  of  death  over  the  body 
and  the  axes  with  reference  to  which  an  order  in  the 
course  of  development,  the  arrangement  of  parts,  or  the 
behavior  of  the  organism  can  be  distinguished.  For  the 
present,  however,  it  must  suffice  to  say  that  the  results 


58  INDIVIDUALITY  IN  ORGANISMS 

of  experimentation  along  this  line  have  demonstrated 
beyond  a  doubt  the  existence  of  such  gradients  as  a 
general  feature  of  the  constitution  of  the  animal  body. 

Such  susceptibility  gradients  may  be  demonstrated, 
not  only  by  the  course  of  death  over  the  body,  but  by 
the  different  degrees  of  retardation  or  inhibition  of 
growth  and  development  at  different  levels  under  the 
same  experimental  conditions.  I  have  described  such 
retardation  or  inhibition  gradients  as  observed  in  the 
flatworm  Planaria,^  and  in  the  development  of  the  sea- 
urchin  I  have  found  it  possible  to  alter  and  control  to 
a  high  degree  the  form  and  proportions  of  the  larva 
through  the  differences  in  susceptibility  along  the  axes 
to  various  reagents.  Such  gradients  are  also  very 
clearly  evident  in  many  cases  described  by  various 
authors  of  the  effect  of  external  conditions  of  various 
kinds  of  development.  The  abnormal  forms  produced 
in  such  experiments  almost  invariably  indicate  the  exist- 
ence of  axial  differences  in  susceptibility.  The  gradient 
which  appears  in  such  cases  is  usually  the  acclima- 
tion gradient,  the  regions  of  highest  metabolic  rate 
being  least  susceptible  and  so  least  affected,  but  if 
the  external  factor  acts  with  sufficient  intensity  or  if 
acclimation  does  not  occur,  the  differences  in  suscepti- 
bility are  parallel  with  the  metabolic  gradient  itself.  In 
the  embryo  of  the  frog,  which  has  been  much  used  for 
experiments  of  this  sort,  various  experimental  conditions 
may  retard  or  inhibit  developmental  processes  in  the 

^  Child,  "Studies  on  the  Dynamics  of  Morphogenesis  and  Inherit- 
ance in  Experimental  Reproduction,  IV,  Certain  Dynamic  Factors  in 
the  Regulatory  Morphogenesis  of  Planaria  dorotocephala  in  Relation 
to  the  Axial  Gradient,"  Jour,  of  Ex  per.  ZooL,  XIII,  1912. 


METABOLIC  GILADIENTS  59 

posterior  region  of  the  body  while  in  the  anterior  region 
development  proceeds  more  or  less  normally.  In  such 
cases  the  posterior  regions,  which  possess  a  lower  meta- 
bolic rate  than  anterior  regions,  do  not  acclimate  to  the 
conditions  as  readily  as  the  latter  and  are  therefore 
retarded  or  inhibited  to  a  greater  extent  in  their  develop- 
ment. Such  embryos  produce  certain  characteristic 
forms  of  monsters,  more  or  less  completely  normal 
anteriorly  and  increasingly  abnormal  in  the  posterior 
direction.  Where  acclimation  does  not  play  a  part  the 
anterior  regions  of  the  embryo  may  be  most,  the  posterior 
least,  affected  and  another  type  of  monsters  results.  In 
many  of  these  monstrous  forms  the  symmetry  gradients 
as  well  as  the  major  gradient  appear  more  or  less  clearly 

In  fact  the  field  of  teratogeny,  the  experimental  pro- 
duction of  monstrous  or  abnormal  forms,  contains  a 
large  amount  of  evidence  for  the  existence  of  suscepti- 
bility gradients,  but  neither  the  relation  between  sus- 
ceptibility and  metabolic  rate  nor  the  existence  of  the 
metabolic  gradients  has  been  recognized  by  the  investi- 
gators in  this  field.  There  is  no  doubt  that  further 
experiments  directly  concerned  with  the  problem  of 
susceptibility  and  metabolic  gradients  will  afford  even 
more  definite  and  positive  results. 

These  gradients  in  susceptibility  indicate  the  ex- 
istence in  the  animal  organism  of  more  or  less  definite 
metabolic  gradients  essentially  quantitative  in  nature. 
In  other  words,  we  find  a  definite  order  in  the  gradation 
of  rate  or  intensity  of  general  metabolic  activity  in 
directions  coinciding  with  those  in  which  an  orderly 
sequence  of  events  and  arrangement  of  parts  or  an 
orderly  behavior  of  the  organism  in  other  respects  are 


6o  INDIVIDUALITY  IN  ORGANISMS 

distinguishable.  Alteration  or  even  reversal  of  certain 
gradients  during  development  in  some  cases  makes  it 
necessary  to  distinguish  between  the  primary  gradients, 
existing  at  the  beginning  or  in  the  early  stages  of  devel- 
opment, and  the  secondary  gradients,  which  arise  by 
alteration  of  the  primary. 

The  primary  relations  between  the  most  conspicuous 
metabolic  gradients  and  the  chief  axes  of  the  individual 
is  briefly  as  follows.  The  major  axis  is  represented  by  a 
gradient  in  which  the  apical  region  is  always  primarily 
the  region  of  highest,  and  the  basal,  that  of  the  lowest, 
rate  of  reaction.  Stated  in  different  terms,  the  region 
of  highest  metabolic  rate  in  this  gradient  always  gives 
rise  in  development  to  the  apical  region  or  head  of  the 
animal,  the  region  of  lowest  rate  to  the  basal  or  posterior 
end.  In  radial  gradients  the  region  of  highest  rate  may 
be  either  peripheral  or  central  according  to  the  character 
of  the  radius.  In  bilaterally  symmetrical  animals  the 
relations  differ  in  different  cases.  In  at  least  most 
bilaterally  symmetrical  invertebrates  the  median  ventral 
region  is  primarily  the  apical  region  of  the  minor  body 
axes,  and  from  this  region  gradients  of  decreasing  rate 
extend  laterally  and  dorsally.  In  the  vertebrates,  on  the 
other  hand,  the  median  dorsal  region  is  primarily  the 
apical  region,  and  gradients  of  decreasing  rate  extend 
laterally  and  centrally.  The  fact  must  be  emphasized 
that  these  are  the  general  and  primary  relations  and  that 
they  may  be  altered  in  various,  but  always  orderly  and 
definite,  ways  during  the  development  of  the  individual. 

These  facts  indicate  very  clearly  that  the  chief  axes 
of  the  animal  body  are  represented  dynamically  by 
metabohc  gradients  and  that  each  organ  or  part  arises 


METABOLIC  GRADIENTS  6l 

in  a  relation  to  one  or  more  of  these  gradients  which  is 
definite  and  characteristic  for  each  kind  of  organism. 
The  relation  of  the  central  nervous  system  to  these 
gradients  is  highly  significant.  The  apical  portion  of 
the  central  nervous  system,  the  cephalic  ganglion  or 
brain,  always  arises  in  the  region  of  highest  metabolic 
rate  in  the  whole  body,  the  apical  region  of  the  major 
axis,  and  such  portions  of  the  central  nervous  system 
as  appear  in  other  parts  of  the  body,  e.g.,  the  longitudinal 
ganglionic  nerve  cords  of  various  invertebrates  and 
the  spinal  cord  of  vertebrates,  always  arise  in  the  regions 
of  highest  rate  in  the  minor  axial  gradients.  In  the 
bilateral  invertebrates  this  is  the  median  ventral,  in 
the  vertebrates  the  median  dorsal,  region.  In  short, 
it  may  be  said  that  where  a  central  nervous  system  is 
present  it  is  the  organ  characteristic  of  the  apical,  i.e., 
the  dominant,  region  in  each  of  the  chief  axial  metabolic 
gradients.  The  functional  dominance  of  the  central 
nervous  system  in  the  later  Kfe  of  the  animal  is  then 
simply  a  more  highly  specialized  expression  of  the 
primary  relation  of  dominance  and  subordination 
existing  at  the  beginning  of  individuation  between 
regions  of  high  and  those  of  lower  metabohc  rate. 

As  regards  plants,  I  have  as  yet  examined  only  some 
fifteen  species  of  marine  algae,  but  in  all  of  these  the 
apical  region  of  each  axis  shows  the  highest  suscepti- 
bility to  the  higher  concentrations  of  cyanides  and  the 
susceptibility  decreases  very  markedly  in  the  basal  direc- 
tion. In  these  plants  there  is  no  such  disintegration  at 
death  as  in  the  lower  animals,  although  in  the  more 
transparent  forms  the  breaking  up  and  coagulation  of 
the  protoplasm  can  be  observed  inside  the  cell.     By  first 


62  INDIVIDUALITY  IN  ORGANISMS 

staining  the  plants  with  neutral  red  and  then  killing 
with  cyanide  or  some  other  reagent  the  susceptibility 
gradient  can  be  made  visible,  for  as  the  cells  die  the 
red  of  the  stain  at  first  becomes  deeper  because  of  in- 
creasing acidity,  then  changes  to  yellow  as  the  alkali  of 
the  solution  enters,  and  finally  all  color  disappears. 

FURTHER  PHYSIOLOGICAL  EVIDENCE   FOR  THE   EXISTENCE 

OF   METABOLIC   GRADIENTS 

The  susceptibility  gradients  do  not  constitute  the 
only  experimental  evidence  for  the  existence  of  meta- 
bolic gradients  in  the  organism.  Estimations  of  carbon- 
dioxide  production  by  means  of  the  Tashiro  biometer,^ 
which  were  made  by  Dr.  Tashiro  at  my  request,  have 
confirmed  the  results  obtained  by  the  susceptibility 
method  in  all  cases  subjected  to  this  test.  The  gradient 
in  carbon-dioxide  production  is  similar  to  the  gradient 
in  metabolic  rate  indicated  by  the  differences  in  sus- 
ceptibility. On  the  other  hand,  in  the  case  of  certain 
nerves  I  have  been  able  to  confirm  Tashiro 's  recent 
discovery  of  a  gradient  in  carbon-dioxide  production  in 
the  direction  of  conduction  of  the  impulse  along  the 
fiber  by  the  demonstration  of  a  gradient  in  susceptibility 
in  the  same  direction,  and  have  found  a  similar  sus- 
ceptibility gradient  in  certain  other  nerves  for  which 
carbon-dioxide  production  has  not  been  determined 
The  gradient  in  the  production  of  carbon  dioxide  indi- 
cates the  existence  of  a  gradient  in  the  rate  or  intensity 
of  the  respiratory  processes,  the  oxidations,  the  region 

'Tashiro,  "A  New  Method  and  Apparatus  for  the  Estimation  of 
Exceedingly  Minute  Quantities  of  Carbon  Dioxide,"  Amer.  Jour,  of 
Physiol.,  XXXII,  1913. 


METABOLIC  GRADIENTS  63 

of  highest  carbon-dioxide  production  being  the  region  of 
highest  respiratory  rate.  Since  the  oxidations  are  un- 
questionably reactions  of  fundamental  importance  in 
the  metabolic  reaction  system,  the  estimations  of 
carbon-dioxide  production  lead  to  the  same  conclusions 
concerning  the  existence  of  metabolic  gradients  as  do 
the  results  obtained  by  the  susceptibility  method. 

So  far  as  technical  and  other  sources  of  error  can  be 
eliminated,  the  rate  of  oxygen  consumption  of  different 
parts  of  the  body  may  be  used  like  the  rate  of  carbon- 
dioxide  production  as  a  measure  of  respiratory  activity. 
The  use  of  this  method  in  animal  physiology  has  been 
such  that  the  data,  while  of  great  value  for  various  other 
purposes,  have  in  most  cases  no  bearing  upon  the  problem 
of  metabolic  gradients.  In  the  plants,  however,  the 
rate  oi  both  oxygen  consumption  and  carbon-dioxide 
production  have  been  found  to  differ  in  different  parts 
in  such  a  way  as  to  indicate  very  clearly  the  existence  in 
the  plant-body  of  metabolic  gradients.  The  growing 
bud,  for  example,  respires  at  a  higher  rate  than  the 
full-grown  stem  or  leaf. 

Differences  in  electrical  potential  indicating  differ- 
ences of  some  kind  in  chemical  or  physical  activity  are 
known  to  occur  very  generally  in  different  parts  of 
both  animal  and  plant  organisms  and  even  in  different 
parts  of  the  same  organ  or  cell.  The  presence  of  these 
electrical  differences  gives  no  clue  to  the  exact  nature 
of  the  physical  or  chemical  differences  which  produce 
them,  but  it  is  becoming  more  and  more  evident  that  in 
both  animals  and  plants  they  are  to  a  large  extent 
associated  with  differences  in  metabolic  activity.  So 
far  as  this  is  the  case,  we  should  expect  in  general  that 


64  INDIVIDUALITY  IN  ORGANISMS 

parts  with  a  higher  respiratory  rate  would  appear  by  the 
usual  methods  as  electro-negative  to  regions  of  lower 
rate. 

Some  twelve  years  ago  Mathews*  observed  a  differ- 
ence in  electrical  potential  along  the  main  axis  of  certain 
simple  animals,  the  hydroids,  the  parts  nearer  the  apical 
end  being  electro-negative  to  those  nearer  the  basal  end. 
In  these  forms  the  susceptibility  method  indicates  that 
the  metabolic  rate  decreases  from  the  apical  toward  the 
basal  end;  that  is,  in  the  same  direction  as  the  decrease 
in  electro-negativity.  Probably  a  similar  electrical 
gradient  exists  in  nerves,  although  in  the  nerves  of  the 
higher  animals  the  change  is  undoubtedly  very  slight 
within  the  physiological  limits  of  length.  As  regards 
the  plants  also  various  data  on  the  differences  of  electric 
potential  suggest  the  existence  of  metabohc  gradients, 
although  the  fact  that  the  observations  were  made  with 
other  objects  in  view  often  leaves  the  evidence  incon- 
clusive as  regards  the  matter  of  gradients. 

In  the  early  stages  of  development  of  the  starfish  I 
have  been  able  to  make  the  axial  metabolic  gradient 
directly  visible  to  the  eye  by  differential  staining  in  the 
living  animal,''  the  stain  in  this  case  consisting  of  a 
colored  precipitate  formed  within  the  cells  by  the  oxida- 
tion of  certain  substances  added  to  the  water.  The  rate 
of  formation  of  this  precipitate  in  different  cells  differs 
with  the  amount  or  activity  of  enzymes  or  other  con- 
ditions which  influence  the  rate  of  oxidation.     In  those 

»A.  P.  Mathews,  "Electrical  Polarity  in  the  Hydroids,"  Amer. 
Jour,  of  Physiol.,  VIII,  1903. 

=*  Child,  "Axial  Gradients  in  the  Early  Development  of  the  Starfish," 
ibid.,XXXYll,  191 5. 


METABOLIC  GRADIENTS  65 

cells  where  the  rate  of  oxidation  is  highest  the  precipitate 
is  formed  most  rapidly  and  vice  versa.  In  the  starfish 
embryos  and  early  larvae  the  precipitate  appears  first  in 
the  cells  of  the  apical  region,  and  a  very  definite  color 
gradient  along  the  main  axis  arises  in  living  animals, 
while  in  animals  which  have  been  killed  before  staining  no 
gradient  appears.  This  method  is  undoubtedly  capable 
of  wide  application. 

These  various  methods  and  results  indicate  the 
possibilities  of  demonstrating  the  existence  of  the  meta- 
boHc  gradients  in  organisms  by  biochemical  and  physio- 
logical methods.  Unquestionably  future  investigation 
will  give  us  much  more  accurate  and  extensive  data  than 
we  possess  at  present. 

EMBRYOLOGICAL   EVIDENCE   FOR   THE   EXISTENCE   OF 
AXIAL  METABOLIC   GRADIENTS 

Gradients  in  rate  of  cell  division,  size  of  cells,  con- 
dition or  amount  of  protoplasm  in  the  cells,  rate  of 
growth,  and  rate  and  sequence  of  differentiation  are 
very  characteristic  features  of  both  animal  and  plant 
development.  Such  gradients  are  definitely  related  to 
the  axes  of  the  individual  or  its  parts,  and  are  evidently 
expressions  of  axial  metabolic  gradients.  While  the 
existence  of  such  gradients  indicates  the  existence  of 
gradients  in  activity  of  some  sort,  the  various  kinds 
of  gradients  are  not  all  necessarily  present  where  meta- 
bolic gradients  exist.  In  some  cases  the  visible  gradient 
may  be  a  gradient  in  rate  of  growth  or  in  protoplasmic 
constitution;  in  still  others  a  gradient  in  sequence  of 
differentiation,  etc.,  and  sometimes  metabolic  gradients 
exist  without  any  structural  indications  of  their  presence. 


66  INDIVIDUALITY  IN  ORGANISMS 

At  best  these  various  kinds  of  gradients  are  merely 
general  indications  of  differences  in  metabolic  rate, 
and  undoubtedly  in  many  cases  the  visible  differences 
along  an  axis  represent  something  more  than  differences 
in  metabolic  rate.  The  important  point  is  that  visible 
indications  of  graded  differences  in  metabolic  rate  occur 
so  generally  in  definite  relations  to  the  chief  axes  of  the 
body. 

In  the  animal  egg  a  gradient  in  the  distribution  of 
the  yolk  is  often  visible  before  development  begins,  and 
in  such  cases  that  part  of  the  egg  which  gives  rise  to  the 
apical  region  of  the  embryo  contains  less  yolk  than  the 
basal  region.'  Associated  with  this  gradient  in  most 
cases  we  find  differences  in  the  size  of  cells  appearing 
in  very  early  embryonic  stages.  In  the  egg  of  the  frog, 
which  is  an  excellent  example  of  this  sort  of  egg,  the 
yolk  gradient  is  very  distinct,  and  the  early  develop- 
mental stages  show  a  gradient  in  the  same  direction  in 
the  rate  of  cell  division  and  the  size  of  the  cells  formed 
(Figs.  lo,  ii).  The  yolk  gradient  and  the  associated 
gradient  in  cell  division  differ  widely  in  different  kinds  of 
eggs:  in  some  cases  only  the  apical  region  of  the  egg 
divides  at  all,  other  parts  serving  as  a  source  of  nutrition 
which  is  gradually  used  up  during  development.  At 
the  other  extreme  are  cases  in  which  no  yolk  gradient  is 
distinguishable  and  differences  in  division  rate  and  size 
of  cells  do  not  become  evident  until  later  stages. 

In  all  cases  developmental  gradients  of  some  sort 
appear  sooner  or  later  as  expressions  of  the  metabolic 
axial  gradients  and  usually  become  more  distinct  as 
morphological  development  proceeds.  The  so-caUed 
law  of  antero-posterior  development  is  a  partial  recogni- 


METABOLIC  GRADIENTS 


67 


tion  of  this  fact.  This  ''law"  is  merely  a  statement 
of  the  observed  fact  that  in  the  development  of  the 
animal  from  the  egg  organs  first  become  morphologically 
\asible  in  that  region  which  becomes  the  anterior  or 
apical  end,  and  from  this  region  morphogenesis  pro- 
ceeds posteriorly  or  basally  in  a  regular,  orderly  manner. 
In  short,  a  gradient  in  morphogenesis  exists  along  the 
major  axis  of  the  body,  the  apical  end  preceding.  In 
addition  to  this  major  gradient  more  or  less  definite 


Figs.  10,  11. — Two  stages  of  cleavage  of  frog's  egg,  showing  axial 
gradient  in  cell  size  resulting  from  gradient  in  rate  of  division. 

morphogenic  gradients  appear  in  relation  not  only  to 
the  minor  axes  of  the  whole  body,  but  also  in  relation  to 
the  axes  of  particular  organs  or  parts.  In  fact  the  law 
of  antero-posterior  development  is  merely  a  statement 
for  the  major  axis  of  the  more  general  law  of  axial 
developmental  gradients 

Embryonic  stages  of  a  flatworm  among  the  inverte- 
brates and  the  chick  among  the  vertebrates  will  serve 
to  show  these  developmental  gradients.  Fig.  12  is  a 
diagrammatic  outline  of  the  adult  stage  of  a  small 
bilaterally    symmetrical    flatworm,    showing    "brain," 


68 


INDIVIDUALITY  IN  ORGANISMS 


pharynx,  and  alimentary  tract;  Fig.  13  is  a  longitudinal 
section,  almost  in  the  median  plane,  of  an  embryo  of  the 
same   species.     The   anterior   end   is   toward   the   left. 


■■^:'^^ 


r<:&..- 


Figs.  12,  13. — Axial  developmental  gradients  in  flatworm,  Plagio- 
stomitm  giradi:  Fig.  12,  outline  of  adiJt  worm,  showing  eyes,  cephalic 
ganglia,  pharj^nx,  and  alimentary  tract  (after  von  Graff);  Fig.  13, 
longitudinal  section  near  median  plane  of  embr>'o,  head  at  left,  showing 
the  apico-basal  or  longitudinal  and  ventro-dorsal  gradients  in  rate  of 
development  (from  Bresslau). 


The  organs  of  the  anterior  end,  the  brain  and  pharynx, 
consist  of  nmnerous  cells,  and  the  morphological  arrange- 
ment is  already  apparent,  while  the  whole  postpharyn- 


METABOLIC  GRADIENTS  6g 

geal  region,  which  in  the  adult  is  by  far  the  larger  part  of 
the  body,  is  very  short  and  consists  of  but  few  cells. 
This  major  gradient  is  very  distinct,  but  the  ventro- 
dorsal gradient  is  also  evident.  The  section  shows 
that  multiplication  of  cells  and  structural  development 
are  proceeding  chiefly  in  the  ventral  region,  while  the 
dorsal  region  consists  of  relatively  few  cells.  Examina- 
tion of  transverse  sections  of  embryos  would  show  the 
transverse  gradients:  we  should  find  that  the  develop- 
ment was  proceeding  more  rapidly  in  the  median  ventral 
region  than  in  the  lateral  regions.  The  transverse 
and  the  ventro-dorsal  gradients  are  in  reality  different 
components  of  the  same  gradient.  The  fact  is  that  a 
developmental  gradient  extends  laterally  and  dorsally 
from  the  median  ventral  region.  In  such  a  bilaterally 
symmetrical  animal  there  are  then  two  chief  develop- 
mental gradients,  a  major,  from  the  anterior  region 
posteriorly,  and  a  minor,  from  the  median  ventral,  or  in 
some  cases  most  of  the  ventral  region,  laterally  and 
dorsally.  In  other  bilaterally  symmetrical  inverte- 
brates relations  are  in  general  similar.  In  Fig.  2  (p.  38) 
the  relations  in  a  simple  case  of  this  sort  are  diagram- 
matically  indicated. 

In  the  vertebrates  the  longitudinal  gradient  is 
similar  to  that  in  the  invertebrates,  but  instead  of  a 
ventro-latero-dorsal  gradient,  as  in  the  invertebrates, 
the  gradient  is  dorso-latero-ventral  in  direction.  Fig.  14 
represents  an  early  stage  of  the  chick  embryo  in  wliich 
the  head  is  just  becoming  morphologically  distinct,  but 
other  organs  are  not  yet  formed,  while  in  Fig.  15,  a 
later  embryonic  stage,  the  head  region  is  advanced 
in    development,    and    differentiation    of    the    body    is 


70 


INDIVIDUALITY  IN  ORGANISMS 


progressing  posteriorly,  the  successive  formation  of  the 
somites  or  segments  being  a  conspicuous  feature  of  this 
progress.     Fig.   i6  is  a  transverse  section  of  an  early 


Figs.  14,  15. — Surface  views  of  two  early  stages  in  embryonic 
development  of  chick,  showing  progress  of  development  in  basal  direction 
from  the  head-region  (upper  end)  and  laterally  from  the  median  region; 
5,  somites  (from  F.  R.  Lillie). 

stage  before  distinct  organs  have  begun  to  form.  At 
this  time  cells  are  separating  from  the  outer  layer  of 
the  body  in  what  will  later  become  the  median  dorsal 


METABOLIC  GILVDIENTS  71 

region,    and   passing   inward    to    form    the   mesoderm. 
Most  of  the  region  of  the  embryo  behind  the  head  in 


['(gr 


^i 


16 


A 


(^1 


'^^ 


17 


Figs.   16,   17. — Transverse  sections  of  chick  embryo  at  ditTcrcnt 
levels,  to  show  developmental  gradients. 

Fig.  14  and  the  extreme  posterior  region  of  the  embr\'o 
in   Fig.    15   are   at   about   this   stage   of   development. 


72  INDIVIDUALITY  IN  ORGANISMS 

Fig.  17  is  a  transverse  section  at,  a  stage  of  development 
corresponding  to  that  attained  at  the  level  of  the  sixth 
somite  of  the  embryo  in  Fig.  15.  At  this  stage  the 
embryonic  nervous  system  is  present  in  the  form  of  a 
tube  open  dorsally,  and  differentiation  has  progressed 
both  laterally  and  ventrally  from  the  median  dorsa! 
region.  In  the  other  vertebrates,  including  the  mam- 
mals, the  developmental  gradients  are  similar. 

Differences  in  rate  of  growth  constitute  another 
feature  of  these  developmental  gradients,  but  the  rela- 
tion between  the  axial  metabolic  gradient  and  rate  of 
growth  is  not  simple,  for  the  period  of  highest  growth 
rate  occurs  at  different  times  in  different  parts  accord- 
ing to  the  time  of  their  formation,  and  it  may  happen 
at  certain  stages  of  development  that  the  rate  of  growth 
at  the  apical  end  of  a  metabolic  gradient  is  lower  than  at 
the  basal,  because  the  region  at  the  apical  end  began 
its  growth  iirst,  has  grown  at  a  more  rapid  rate,  and  is 
therefore  completing  its  growth  earlier  than  the  region 
at  the  basal  end.  Nevertheless,  so  far  as  it  is  possible 
to  compare  corresponding  stages  in  the  development  of 
different  parts,  along  an  axial  gradient,  differences  in 
rate  of  growth  corresponding  to  the  gradient  do  appear. 
The  head-region,  for  example,  at  the  stage  of  highest, 
growth  rate  grows  more  rapidly  than  the  posterior 
region  of  the  body  at  its  stage  of  highest  rate,  and 
similar  relations  exist  with  reference  to  other  gradients. 

In  the  egg  of  the  plant  as  well  as  in  that  of  the  animal 
developmental  gradients  usually  appear  in  early  stages. 
In  the  eggs  of  many  of  the  lower  plants  the  first  division 
is  transverse,  the  two  cells  thus  formed  representing 
apical  and  basal  regions  of  the  plant,  and  in  most  of  the 


METABOLIC  GRADIENTS 


73 


plant  groups  a  more  or  less  definite  relation  exists 
between  the  directions  o(  the  early  divisions  and  the 
major  axis  of  the  embryo.  In  these  cases  a  more  or  less 
distinct  gradient  in  division  rate,  cell  size,  and  cellular 
constitution  usually  appears  either  at  the  beginning  of 
development  or  in  early  stages.  On  the  one  hand,  this 
gradient  shows  a  definite  relation  to  the  position  of  the 
Qgg  with  respect  to  surrounding  parts  of  the  parent  or- 
ganism, and,  on  the  other,  the  region  of  smallest  size  and 
most  rapid  division  of  the  cells  and  most  abundant  and 
deeply  staining  protoplasm  is  the  region  of  highest  rate 
of  reaction  and  becomes  the  apical  region  of  the  embryo. 
Fig.  1 8  shows  this  gradient  in  the  embryo  of  a  moss,  the 
uppermost  cell  in  the  figure  representing  the  apical  region 
of  the  embryo. 

In  most  of  the  higher  plants  only  a  portion  of  the  egg 
takes  part  in  the  formation  of  the  embryo,  the  remain- 
der forming  a  suspensor,  a  stalk  on  which  the  embryo 
is  carried.  Fig.  19  shows  the  cellular  gradient  in  the 
early  developmental  stage  known  as  the  proembryo 
of  Ginkgo,  a  gymnosperm  related  to  the  conifers.  The 
embryo  proper  arises  later  from  the  small-celled  tissue 
in  the  lower  part  of  the  developing  egg.  Some  of  the 
cycads  also  show  a  very  definite  gradient  of  this  sort. 
In  the  angiosperms,  the  higher  seed  plants,  where  the 
egg  is  attached  to  the  wall  of  the  embryo-sac,  the  embryo 
arises  from  its  free  apical  end. 

A  characteristic  feature  of  the  plant  individual  in  all 
except  the  simplest  forms  is  the  growing  or  vegetative 
tip.  This  growing  tip  is  the  region  of  most  active 
nuclear  division  and  growth  and  with  rare  exceptions 
forms  the  free  end  of  the  individual  and  gives  rise  to 


74 


INDI\  IDUALITY  IN  ORGANISIVIS 


other  parts  of  the  plant  body.  In  the  complex  higher 
plant,  stems,  branches,  buds,  roots,  and  various  other 
parts  possess  a  growing  tip,  at  least  during  earlier  stages, 
and  each  such  part  is  to  a  certain  extent  an  individual. 


Figs.  i8,  19. — Axial  developmental  gradients  in  embryonic  stages 
of  plants:  Fig.  18,  embryo  of  moss,  apical  cell  at  upper  end  (from  prepara- 
tion loaned  by  W.  J.  G.  Land);  Fig.  19,  proembryo  of  g>'mnosperm 
(Ginkgo);  apical  region  of  plant  arises  from  lower  end  (from  Lyon). 


In  most  of  the  lower  plants  a  single  cell  forms  the  apex 
or  center  of  the  growing  tip,  and  it  may  be  larger  than  ' 
other  cells  with  a  gradient  of  decreasing  size  extending 
from  it,  as  in  the  stem  of  the  alga  in  Fig.  20,  but  during  the 
course    of    plant    evolution    the    apical    cell    gradually 


METABOLIC  GRADIENTS 


75 


gives  place  to  an  apical  region,  consisting  of  several  or 
many  cells,  and  in  the  course  of  this  change  the  apical 
cell  itself  becomes  relatively  smaller,  and  a  gradient  of 
increasing  size  extends  from  the  apical  region  (Figs.  i8, 
36).     The  gradients  in  size  in  different  forms  depend 


Fig.   20. — Axial  gradient  in  ceU  size  in  alga  Cladostephiis  (from 
Pringsheim) . 


upon  the  relation  between  frequency  of  division  and 
growth  in  size  of  the  apical  cell,  and  this  relation  shows  a 
characteristic  range  in  each  form.  Even  where  the 
whole  plant  body  is  a  single  multinucleate  cell,  the 
apical  regions  of  stem  and  branches  are  undoubtedly 
physiologically  growing  tips.     In  the  higher  plants  the 


76 


INDIVIDUALITY  IN  ORGANISMS 


growing  tip  consists  of  several  or  many  cells.  Figs.  21 
and  22,  longitudinal  sections  through  the  growing  tips  of 
a  stem  and  a  root  respectively,  show  the  gradients  in  cell 

m 


Figs.  21,  22. — Axial  developmental  gradients  in  growing  tips  of 
seed  plants:  Fig.  21,  stem-tip  of  Hippuris;  Fig.  22,  root-tip  of  Trades- 
cantia  (from  preparations  loaned  by  Department  of  Botany,  University 
of  Chicago). 

size  and  protoplasmic  condition  which  extend  from  the 
growing  tips.  In  the  stem-tip  these  gradients  extend  to 
a  much  greater  distance  than  in  the  root- tip  and  Fig.  21 
shows  only  a  fraction  of  them. 


METABOLIC  GRADIENTS  77 

In  the  development  of  the  plant  the  growing  tip  is 
the  first  part  of  the  individual  to  become  distinguishable, 
and  from  it  other  parts  arise.  In  the  moss  em]:)ryo  in 
Fig.  18  the  growing  tip  is  already  present  as  the  upper- 
most cell  and  other  cells  have  arisen  from  it  in  an  orderly 
way.  In  the  higher  plants  the  growing  tip  is  not  usually 
localized  until  later  stages.  In  Gingko,  for  example,  the 
growing  tip  of  the  plant  is  not  yet  distinguishable  at 
the  stage  of  Fig.  19,  although  the  small-celled  region  is 
the  growing  tip  of  the  whole  proembryo  and  in  this  the 
growing  tip  of  the  plant-stem  later  appears.  In  certain 
algae  the  major  axial  gradient  in  the  egg  is  apparently 
determined  by  external  factors,  such  as  light,  but  in 
most  plants  this  gradient  is  determined  by  the  relation  of 
the  egg  to  the  parent  body,  the  growing  tip  of  the  plant- 
stem  arising  from  the  apical  region  of  this  gradient. 

The  vegetative  stages  of  certain  liverworts  and  the 
sexual  generation  of  various  ferns  show  a  high  degree  of 
bilateral  symmetry  and  often  consist,  at  least  during  the 
earlier  stages  of  their  growth,  of  single  elongated  flattened 
individuals  (Figs.  23,  24)  with  a  growing  tip,  a,  at  one 
end,  often  with  a  thickened  longitudinal  midrib  and  with 
root-like  outgrowths  on  the  ventral  surface,  the  surface 
facing  the  substratum  as  the  plant  grows.  In  many 
cases  these  individuals  undergo  division  by  branching  or 
by  the  formation  of  buds  on  the  surface  in  later  stages. 
In  these  plants,  as  in  bilaterally  symmetrical  animals, 
three  axes — longitudinal,  transverse,  and  dorso-ventral — 
are  distinguishable;  in  other  words,  order  is  apparent  in 
three  directions.  Various  indications  of  gradients  in 
activity  appear  in  the  same  directions.  As  regards  the 
major  axis,  the  rate  of  cell  division  and  growth  is  highest 


78 


INDIVIDUALITY  IN  ORGANISMS 


in  the  apical  region  and  decreases  basally;  as  the  plant 
grows  older,  death  may  even  begin  at  the  basal  end  and 
proceed  apically  while  the  apical  end  is  still  growing 
actively.  Evidences  of  a  transverse  gradient  in  activity 
appear  in  a  decrease  in  growth  toward  the  lateral  margins 
and  in  many  forms  in  a  decrease  in  thickness  of  the 
body  in  the  same  direction.     In  the  direction  of  the 


Figs.    23,    24. — Bilaterally    symmetrical    prothallia    of   lix^erwort, 
Marchanlia  (dorsal  view),  and  a  fern  (ventral  view). 

dorso-ventral  axis  which  is  determined  by  the  action  of 
light  and  perhaps  other  external  factors,  the  differences 
in  metabolic  activity  are  indicated  by  the  outgrowth  of 
root-like  structures  and  the  sexual  organs,  and  in  some 
forms  of  scales  or  leaf-lilve  structures  on  the  ventral 
surface,  and  also  in  some  forms  by  the  greater  density 
of  cellular  structure  iri  the  ventral  region. 


METABOLIC  GRADIENTS  79 

DEVELOPMENTAL   GRADIENTS    IN   AGAMIC   AND 
EXPERIMENTAL   REPRODUCTION 

Among  the  lower  animals  and  most  plants  new  indi- 
viduals arise,  not  only  by  the  process  of  gametic  or  sexual 
reproduction,  but  by  various  agamic  or  asexual  pro- 
cesses, such  as  division,  budding,  etc.  These  processes 
vary  greatly  in  different  forms  and  even  in  the  same 
individual  under  different  conditions,  but  their  essential 
feature  is  the  formation  of  a  new  individual  from  a  part 
of  a  pre-existing  individual,  a  process  which  usually  in- 
volves more  or  less  dedifferentiation  and  redifferentiation 
in  a  new  direction.  Although  these  agamic  reproductive 
processes  differ  more  or  less  widely  from  embryonic 
development,  the  metabolic  gradients  characteristic 
of  the  individual  either  persists  from  the  original  indi- 
vidual or  arise  anew  in  each  case,  and  developmental 
gradients  of  some  sort  appear  in  relation  to  them. 

In  the  formation  among  animals  of  new  individuals 
by  budding,  as,  for  example,  in  the  hydroid,  Pennaria 
(Figs.  25-27),  the  hydranth  becomes  distinguishable 
first,  the  stem  later,  and  closer  examination  shows  that 
apical  regions  of  the  hydranth  are  somewhat  in  advance 
of  basal.  In  Figs.  26  and  27,  for  example,  the  apical 
tentacles  are  more  advanced  in  development  than  the 
basal. 

In  the  flatworm,  Stenostomum,  division  occurs  after 
the  body  attains  a  certain  length,  the  first  visible  indica- 
tion of  the  new  individual  being  the  appearance  of  a  new 
head-region  (Fig.  28)  at  a  certain  distance  from  the 
original  head.  This  new  head-region  acquires  control 
of  parts  posterior  to  it  and  finally  separates  as  a  new 
animal.     By    continued    division    before    separation    of 


8o 


INDIVIDUALITY  IN  ORGANISMS 


each  new  individual  thus  formed  chains  of  from  eight 
to  sixteen  individuals  or  zooids,  as  they  are  usually 
called,   in  various   stages  of  development  may   result 


Figs.  25-27. — Pennaria  tiarella:    Fig.  25,  h,  h\  h",  Figs.  26  and 
27,  stages  of  development  of  hydranth;  m,  medusa  bud. 

(Fig.  29).     Many  other  cases  of  division  among  animals 
are  essentially  similar. 

In  many  of  the  lower  animals  agamic  reproduction 
can  be  induced  experimentally  by  isolating  pieces.     In 


METABOLIC  GRADIENTS 


8i 


A 


the  flatworm  Planaria  (Fig.  30)  a  piece  such  as  a  or  b, 
or  almost  any  other  piece,  cut  from 
the  body  will  develop  into  a  whole 
animal  of  small  size  by  the  forma- 
tion of  a  new  head  at  one  end  and  a 
new  tail  at  the  other  and  a  trans- 
formation and  redifferentiation  of 
the  internal  organs  of  the  piece  into 
those  of  a  whole  animal  as  indi- 
cated in  Figs.  31-33.  In  the  out- 
growth of  the  new  tissue  at  the 
two  cut  surfaces  the  axial  gradients 
appear    as    gradients    in    rate    of 

growth.     Fig.   31   shows   that  the 

outgrowth  of  new  tissue  is  more 

rapid   at   the  apical   than  at   the 

basal  end  of  the  piece  and  more 

rapid  in  the  median  than  in  the 

lateral  region  of  each  cut  surface, 

and  Fig.  34,  a  side  view  of  the 

piece,  shows  more  rapid  outgrowth 

at  each  end  in  the  ventral  than  in 

the  dorsal  region.     In  this  case  the 

axial  gradients  in  the  piece  persist 

from  the  parent  individual,  and  the 

head  arises  at  the  apical  end  of 

the  piece,  the  tail  at  the  basal  end. 

In    other    cases    of    experimental 

reproduction   from  isolated  pieces 

the  axial  gradients  appear  either 

in  the  same  or  in  some  other  way 

according  to  the  kind  of  individual  and  the  conditions. 


Figs.  28,  29. — Asexual 
reproduction  in  llat- 
worm,  Stcnostomum: 
Fig.  28,  stage  of  two 
zooids;  Fig.  29,  chain  of 
several  zooids. 


82 


INDIVIDUALITY  IN  ORGANISMS 


In  agamic  reproduction  in  plants  each  new  individual 
arises  as  a  localized  region  of  growth  and  the  growing 
tip  is  the  first  region  to  become  clearly  defined.     New 


'^i\: 


a 


31 


'A 


-OJl 


33 


34 


Figs.  30-34. — Planaria  doroto- 
cephala:  Fig.  30,  structure  of  ali- 
mentary tract  and  arrangement  of 
central  nervous  system;  a,  b,  two 
regions  indicating  pieces  for  reconsti- 
tution;  Figs.  31-33,  stages  of  reconsti- 
tution;  Fig.  34,  side  view  of  early 
stage. 


buds,  new  roots,  and  other  parts  arise  in  this  way  in 
nature  and  under  experimental  conditions.  The  small 
outgrowths  along  the  sides  of  the  growing  stem-tip  in 


METABOLIC  GRADIENTS  83 

Fig.  21  (p.  76)  are  stages  in  the  formation  of  leaves  and 
the  developmental  gradients  appear  to  some  extent  in 
them,. 

In  many  plants  new  "adventitious"  individuals 
arise,  either  in  nature  or  under  experimental  conditions, 
from  cells  already  difTerentiated  as  part  of  an  individual. 
In  the  liverwort,  Melzgeria,  new  individuals  may  arise 
either  by  division  of  the  growing  tip  resulting  in  bifurca- 
tion of  the  flat  body,  as  shown  in  Fig.  35,  a,  a,  or  after 
injury  to,  or  removal  of,  the  growing  tip  by  a  renewal  of 
division  and  growth  in  dilTerentiated  cells.  Fig.  36 
shows  the  cellular  structure  of  the  growing  tip  in  a 
well-developed  individual  and  Fig.  37  the  early  stage 
of  a  new  individual  formed  from  a  differentiated 
cell.  In  both  figures  the  gradient  in  cell  size  is  clearly 
evident. 

Among  the  higher  seed  plants,  as  well  as  among  lower 
forms,  the  "adventitious"  formation  of  new  individuals 
from  differentiated  cells  occurs,  as  for  example  in  the 
begonias,  where  buds  capable  of  producing  new  plants 
arise  under  certain  experimental  and  natural  conditions 
from  the  epidermal  cells  of  leaves.  The  epidermal 
cells  which  take  part  in  the  formation  of  such  a  bud 
lose  their  differentiated,  vacuolated  condition,  become 
filled  with  protoplasm,  like  embryonic  cells,  and  divide 
rapidly.  Fig.  38  is  a  surface  view  of  the  formation  of 
such  a  bud  involving  several  epidermal  cells,  but  centered 
chiefly  in  parts  of  four  cells,  and  Fig.  39  is  a  longitudinal 
section  through  a  bud  formed  from  two  cells  The 
double  contours  in  Fig.  38  show  the  thickened  cellulose 
walls  of  the  original  epidermal  cells,  the  single  contours 
within  them  the  cells  formed  by  their  repeated  division, 


84 


INDIVIDUALITY  IN  ORGANISMS 


and  the  shading  indicates  in  a  general  way  the  disap- 
pearance of  the  vacuoles  and  the  filling  of  the  cells  with 


a 


37 


Figs.  35-37. — Metzgeria,  a  liverwort:  Fig.  35,  portion  of  pro- 
thallium,  showing  midrib  and  apical  regions,  a,  a;  Fig.  36,  cell  structure 
of  growing  tip,  showing  apical  cell,  a,  and  gradient  in  cell  size;  Fig.  37, 
cell  structure  of  an  adventitious  bud,  showing  apical  cell,  a,  and  gradient 
in  cell  size  (Figs.  36  and  37  from  Goebel). 

protoplasm.  A  gradient  in  cell  size  and  protoplasmic 
condition  appears  in  both  cases,  in  Fig.  -^Z  from  the 
center  to  the  periphery  of  the  region  involved  and  in 


METABOLIC  GRADIENTS  8- 

Fig.  39  from  the  upper  part  at  the  free  surface  of  the 
leaf    downward.     These    gradients   are    evidently    the 


Figs.  38-41. — Origin  of  adventitious  buds  in  seed  plants:  Fig.  3S, 
surface  view  and  Fig.  39,  section  of  bud  arising  from  dilTerentiated  epi- 
dermal cells  of  leaf  of  Begonia  (from  Regel);  Figs.  40,  41,  development 
of  bud  in  callus  (from  Simon). 

visible  expression  of  gradients  in  metabolic  activity,  the 
smallest,  most  protoplasmic  cells  indicating  the  region  of 
most  intense  activity,  and  it  is  from  tliis  most  active 


86  INDIVIDUALITY  IN  ORGANISMS 

region  that  the  apical  vegetative  tip  of  the  new  plant 
individual  develops. 

*In  many  woody  plants  the  cut  end  of  a  stem  or 
branch  develops  a  mass  of  wound  tissue,  the  callus,  and 
in  this  callus  new  buds  arise  independently  of  other 
parts  of  the  plant  and  become  connected  with  them 
secondarily.  In  all  such  cases  the  differentiation  of  the 
vascular  bundles  which  connect  the  new  buds  with  the 
old  parts  proceeds  from  the  buds.  Fig.  40  shows  an 
early  stage  of  bud-formation  in  the  poplar  at  the  periph- 
ery of  a  mass  of  callus  on  the  cut  end  of  a  stem,  and 
Fig.  41,  a  later  stage  in  which  vascular  connection  with 
other  parts  has  been  established.  In  such  cases  the 
appearance  of  the  new  bud  is  the  first  step  in  the  forma- 
tion of  the  new  individual ;  it  is  followed  by  the  appear- 
ance of  a  gradient  in  growth  and  differentiation  from  the 
bud  toward  other  parts. 

In  isolated  pieces  of  plants  the  formation  of  new 
growing  tips  or  the  outgrowth  of  resting  buds  occurs 
in  certain  more  or  less  definite  portions  with  relation 
to  the  axes.  The  removal  of  the  chief  growing  tip  of  a 
stem  results  in  outgrowth  or  altered  growth  of  the 
uppermost  buds  or  branches.  When  these  are  removed 
those  lower  down  react,  and  so  on.  Evidently  a  gradient 
in  the  capacity  to  respond  or  in  the  rate  of  response  to 
the  altered  conditions  exists  along  the  major  axis,  and 
those  buds  or  branches  which  react  first  dominate  those 
below  them  and  prevent  them  from  reacting  in  the 
same  way. 

In  isolated  pieces  of  the  bilaterally  symmetrical 
liverworts,  such  as  Marchantia  (Fig.  23,  p.  78),  the 
position  of  the  new  buds  evidently  represents  the  region 


METABOLIC  GRADIENTS  87 

of  highest  metabolic  rate  in  the  piece  as  a  resultant 
of  the  three  axial  gradients  (see  Figs.  99-102,  p.  167), 
and  the  formation  of  new  individuals  in  these  regions 
inhibits  their  formation  elsewhere,  although  practically 
every  cell  of  the  plant-body  is  capable  under  proper 
conditions  of  giving  rise  to  a  new  individual. 

CONCLUSION 

All  the  various  lines  of  evidence  considered  agree  in 
showing  that  axial  gradients  in  the  dynamic  processes 
are  characteristic  features  of  organisms  and  that  a 
definite  relation  exists  in  each  individual  between  the 
direction  of  the  gradient  in  any  axis  and  the  physiological 
and  structural  order  which  arises  along  that  axis.  In 
the  major  axis  the  region  of  highest  rate  in  the  metabolic 
gradient  becomes  the  apical  or  anterior  region  of  the 
individual,  and  in  the  minor  axes  also  the  regions  of 
highest  rate  in  the  gradients  represent  particular  features 
of  the  order  in  each  case.  Along  any  axis  particular 
parts  apparently  represent  particular  levels  in  the 
gradients.  The  variety,  extent,  and  agreement  of  the 
evidence  is  all  the  more  interesting  in  view  of  the  fact 
that  such  gradients  have  not  heretofore  been  recognized 
2VS  characteristic  features  of  organic  constitution. 


CHAPTER  IV 

PHYSIOLOGICAL  DOMINANCE  IN  THE  PROCESS 
OF  INDIVIDUATION 

According  to  the  theory  outlined  in  chap,  ii,  the 
organic  individual  is  fundamentally  a  dynamic  relation 
of  dominance  and  subordination,  associated  with  and 
resulting  from  the  establishment  of  a  metabolic  gradient 
or  gradients.  In  the  present  chapter  some  of  the  evi- 
dence for  the  existence  of  dominance  in  the  process  of 
individuation  is  considered. 

This  evidence  is  obtained  primarily  from  the  experi- 
mental reproductions,  because  only  here  is  it  possible  to 
analyze  and  control  the  process  of  individuation  to  any 
considerable  degree.  The  egg  is  usually  a  more  or  less 
highly  specialized  individual  at  the  time  embryonic  de- 
velopment begins,  and  the  earlier  stages  of  its  individ- 
uation commonly  occur  in  such  relations  to  the  parent 
body  that  they  are  not  readily  accessible  to  experimental 
investigation.  Nevertheless,  the  evidence  indicates  very 
clearly  that  the  process  of  organic  individuation  is  fun- 
damentally the  same  in  the  egg  and  embryo  and  in  ex- 
perimental reproduction. 

The  evidence  presented  here  concerns  primarily  the 
major  axis,  because  the  facts  are  simpler  and  more  com- 
plete with  respect  to  this  axis.  Experimental  isolation 
of  pieces  with  reference  to  the  minor  axes  is  usually 
complicated  by  the  presence  of  the  major  gradient,  and 
the  order  along  the  major  axis  is  often  such  that  parts 
necessary  for   continued  life  are  absent  from   various 

88 


PHYSIOLOGICAL  DOMINANCE  8g 

regions  of  the  minor  axes.  For  these  reasons  the  experi- 
mental investigation  of  dominance  and  subordination 
in  relation  to  the  minor  axes  is  variously  complicated 
and  limited  in  different  cases.  Nevertheless,  the  funda- 
mental similarity  of  the  different  directions  of  order  in 
the  individual  is  indicated  by  various  lines  of  evidence, 
and  there  are  no  grounds  for  hesitation  in  extending  to 
the  minor  axes  general  conclusions  reached  concerning 
the  major  axes. 

THE   EXPERIMENTAL   MATERIAL 

Reproduction  can  be  induced  experimentally  in  the 
plants  and  many  of  the  lower  animals  by  the  isolation 
of  pieces  and  in  various  other  ways.  These  experimental 
reproductions,  when  properly  controlled  and  analyzed, 
constitute  invaluable  material  for  study  of  the  problem 
of  the  individual,  for  it  is  often  possible  to  increase  or 
decrease  dominance  and  so  to  extend  or  decrease  its 
range,  to  alter  the  conductivity  of  protoplasm,  to 
determine  the  elimination  of  old  and  the  establishment 
of  new  metabolic  gradients,  and  in  these  and  other 
ways  to  control  the  process  of  individuation  to  some 
extent,  and  to  determine  the  results  of  such  control 

Most  plants  and  many  of  the  lower  animals  give 
rise  to  new  individuals  by  division,  budding,  and  other 
agamic  processes,  and  the  new  indi\iduals  thus  formed 
often  remain  organically  connected  and  give  rise  to  a 
composite  individual,  such  as  a  tree  among  plants  or  a 
hydroid  colony  among  animals.  In  such  reproductions 
definite  and  orderly  space  or  distance  relations  are 
observable,  which  themselves  suggest  the  existence  of  a 
limited  range  of  dominance.     The  occurrence  of  division 


90 


INDI\aDUALITY  IN  ORGANISMS 


when  a  certain  size  or  length  is 
attained,  or  the  appearance  of 
buds  at  a  certain  distance  from 
the  chief  growing  tip  in  plants,  are 
cases  in  point.  In  many  cases  ex- 
perimental control  and  alteration 
of  these  relations  throw  a  flood  of 
light  upon  the  problem  of  their 
nature.  It  is  with  material  and 
experiments  of  this  sort  that  the 
present  chapter  is  largely 
concerned. 

I  have  shown  elsewhere  that 
the  process  of  progressive  develop- 
ment and  differentiation  in  the 
individual  is  accompanied  by  a 
decrease  in  the  metabolic  rate 
determined  by  the  accumulation 
of  relatively  inactive  constituents 
in  the  protoplasm.     These  changes, 

which     constitute 


JL 


Figs,  42,  43. — Tuhiilaria:  Fig.  42,  a  single 
individual;  Fig.  43,  asexual  reproduction 
from  tip  of  stolon. 


senescence,  may 
end  in  death  if 
they  go  far  enough. 
On  the  other  hand, 
any  change  which 
brings  about  the 
removal  of  such 
previously  accu- 
mulated material 
makes  possible  an 


acceleration  in  metabolic  rate,  and  such  changes  con- 


PHYSIOLOGICAL  DOMINANCE  91 

stitute  rejuvenescence.  The  facts  indicate  that  all 
reproductive  processes  bring  about  rejuvenescence  to 
some  degree,  and  it  is  certain  that  the  new  indi- 
viduals which  arise  by  division  or  budding  from  other 
individuals  or  from  experimentally  isolated  pieces  are 
to  some  extent  physiologically  younger  than  the  parent 
individual  from  which  they  arose/  Rejuvenescence  in 
such  cases  results  from  the  loss  of  the  differentiation  as 
a  part  in  that  portion  concerned  in  the  reproductive 
process,  and  with  the  new  individuation  a  new  process 
ot  senescence  begins. 

Among  the  lower  animals  which  have  served  as 
material  for  the  study  of  regeneration  or  regulation 
two  forms  have  been  used  to  a  large  extent  in  my  own 
experiments  and  must  be  briefly  described  here.  The 
hydroid  Tuhularia  in  its  simple  unbranched  form  as 
a  single  individual  (Fig,  42)  consists  of  hydranth,  stem, 
and  stolon,  the  hydranth  forming  the  apical  end  of  the 
stem  and  bearing  two  sets  of  tentacles,  reproductive 
organs  between  them,  and  a  mouth  at  its  apical  end. 
The  stem  grows  vertically  from  the  surface  of  attach- 
ment, and  the  stolon  adheres  to  the  surface,  forming  an 
organ  of  attachment,  and  elongates  by  growth  at  its 
tip„  Stem  and  stolon  are  covered  by  a  horny  cuticle, 
the  perisarc.  The  apical  end  of  the  metabolic  gradient 
of  the  major  axis  is  the  apical  region  of  the  hydranth,  and 
from  this  region  the  rate  decreases  basally  through  tht* 
hydranth.  In  the  stem  the  metabolic  rate  is  lower  than 
in  the  hydranth,  and  there  is  a  slight  decrease  in  rate  in 
the  basal  direction,  but  at  the  growing  tip  of  the  stolon 
there  is  a  short,  slight  gradient  in  the  opposite  direction. 

'  Child,  Senescence  and  Rejuvenescence,  191 1;. 


92  INDIVIDUALITY  IN  ORGANISMS 

The  primary  form  of  asexual  reproduction  in  Tubu- 
laria  is  represented  in  Fig.  43.  When  the  stem  and 
stolon  together  attain  a  certain  length,  which  varies  with 
the  metabolic  condition  of  the  animal  but  under  favor- 
able conditions  may  be  five  to  eight  centimeters,  the 
stolon  turns  away  from  the  substratum  and  gives  rise  to  a 
hydranth;  then  a  stem  forms  and  elongates  below  this 
hydranth,  and  a  new  stolon  arises  from  the  base  of  this 
stem.  This  process  of  reproduction  itself  suggests  that 
the  tip  of  the  stolon  is  subordinate  to  the  original 
hydranth  until  it  attains  a  certain  distance  from  it  and 
then  is  able  to  produce  a  new  hydranth,  and  experi- 
ments show  that  this  is  true.  If  the  original  stem 
elongates  still  further  new  hydranths  may  arise  along 
the  stolon  and  at  the  base  of  the  stem,  as  these  regions 
become  physiologically  isolated. 

In  Corymorpha,  a  form  related  to  Tuhularia,  the 
hydranth  is  much  larger,  the  stem  naked  except  near 
the  base  and  reaching  a  length  of  ten  to  twelve  centi- 
meters, and  instead  of  a  stolon  the  basal  end  is  imbed- 
ded in  sand  and  bears  delicate  root-like  outgrowths  as 
holdfasts  (see  Figs.  74,  78,  pp.  143,  145). 

Planaria  dorotocephala  (Fig.  30,  p.  82),  a  flatworm 
and  one  of  a  number  of  species  much  used  in  experiment, 
is  a  much  more  highly  differentiated,  bilaterally  sym- 
metrical form,  with  distinct  head  and  "brain"  and  two 
ventraJ  nerve  cords,  and  with  definite,  though  rather 
diffuse,  alimentary  and  excretory  organs.  Sexual  organs 
appear  in  this  form  only  under  certain  conditions.  This, 
as  well  as  various  other  species  of  the  group,  undergoes 
fission  after  it  attains  a  certain  variable  size,  the  separa- 
tion usually  occurring  at  about  the  level  f  in  Fig.  44 


PHYSIOLOGICAL  DOMINANCE 


93 


The  separated  posterior  portion  becomes  a  new  animal, 
while  the  anterior  portion  develops 
a  new  posterior  end,  and  fission  is 
sooner  or  later  repeated.  There  is 
no  morphological  indication  of  a 
second  individual  or  zooid  in  the 
posterior  region  of  the  body,  but 
one  or  more  such  individuals  are 
indicated  by  the  metabolic  gradient 
of  the  major  axis  and  by  various 
other  physiological  differences. 
The  apical  region  of  this  gradient 
is  the  head  of  the  animal,  and  from 
the  head  the  metabolic  rate  de- 
creases to  the  level  where  separa- 
tion occurs  in  fission;  there  a 
sudden  rise  in  rate  occurs,  and  then 
again  a  downward  gradient  toward 
the  posterior  end.  The  region 
where  the  rate  rises  suddenly 
represents  the  apical  end  of  the 
second  individual  and  the  down- 
ward gradient  following  is  the 
gradient  of  the  major  axis  of  this 
zooid.  In  the  shorter  animals 
only  one  of  these  zooids  is  present, 
but  as  the  length  increases  the 
basal  body  region  may  show  two, 
three,  or  more  of  these  distinct  gra- 
dients. Represented  graphically 
the  metabolic  gradient  in  such  an 
animal  is  like  the  curve  in  Fig.  45;  a  is  the  head-region, 


/ 


Fig.  44. — Planaria 
dorotoceplujla,  outline, 
indicating  several  zooids 
in  basal  region;  ff,  usual 
level  of  fission. 


94  INDIVIDUALITY  IN  ORGANISMS 

the  long  slope  the  body  of  the  anterior  chief  zooid,  which 
forms  most  of  the  body  of  the  worm,  h  represents  the 
apical  end  of  the  second  zooid,  c  that  of  a  third,  etc. 
These  zooids  are  the  result  of  successive  physiological 
isolations  of  the  basal  region  as  the  animal  grows  in 
length.  First  a  single  zooid  is  formed  at  the  basal  end, 
but  the  range  of  dominance  is  short  in  this  undeveloped 
individual,  and  as  growth  proceeds  its  basal 
region  soon  becomes  physiologically  isolated, 
and  a  second  zooid  arises,  and  so  on.  While 
the    degree    of    physiological    isolation    is    not 


Anterior  Posterior 

Fig.  45. — Graphic  representation  of  major  axial  gradients  in  a 
Planar ia  with  several  zooids:  a,  head  of  animal;  b,  c,  apical  regions 
of  secondary  zooids. 


sufficient  to  permit  the  development  of  the  new  indi- 
vidual to  proceed  very  far,  some  degree  of  rejuve- 
nescence in  the  part  does  occur  and  its  metabolic  rate 
rises  slightly,  and  with  each  successive  isolation  there  is 
a  further  increase  in  rate,  so  that  in  each  successive 
zooid  the  gradient  is  at  a  level  somewhat  higher  than 
that  of  the  preceding. 


PHYSIOLOGICAL  DO]MINANCE 


95 


The  act  of  fission  in  this  animal  consists  of  an  indc' 
pendent  motor  reaction  of  the  posterior  zooid  or  group 
When  the  worm  is  creeping  quietly, 
the   posterior   zooid    or    the    zooid 
group  suddenly  attaches  itself  to  the 
surface    on    which    the    animal    is 
creeping,  wliile  the  whole  anterior 
individual    endeavors    to    advance 
and  the  body  in  front  of  the  attached 
region    becomes    greatly    stretched 
(Fig.  46)  and  finally  ruptures.     The 
occurrence  of  fission  can  often  be 
controlled  experimentally  in  a  way 
that   shows   the   variable   range   of 
dominance  very  clearly.     If  an  ani- 
mal is  very  slightly  stimulated,  e.g., 
by  a  slight  jarring  of  the  aquarium, 
the  posterior  zooid  will  often  attach 
itself,  and  fission  will  occur,  while 
with  stronger  stimulation  the  animal 
is  able  to  control  this  region  and  it 
does  not  become  attached  but  ad- 
vances with  the  rest  of  the  body! 
Evidently  when  the  animal  is  only 
moderately  active  the  posterior  re- 
gion is  physiologically  isolated,  but 
when  it  is  intensely  active  the  range 
of  dominance  of  the  anterior  indi- 
vidual   extends    to    this    posterior 
region  and  determines  its  subordina- 
tion in  behavior.     Similarly,  in  very  old  animals  which 
have  been  prevented  from  undergoing  fission  by  keeping 


Fig.  46. — Pldftaria 
dorotoccpJmla  in  the  act 
of  fission. 


96  INDIVIDUALITY  IN  ORGANISMS 

them  on  a  layer  of  vaseline  or  other  surface  to  which  they 
cannot  attach  themselves,  the  tissues  are  often  so  tough 
that  rupture  does  not  readily  occur,  and  the  anterior 
indi\adual  struggles  more  and  more  violently  to  free 
itself  from  the  hindrance  which  is  preventing  its  advance. 
In  these  animals  such  struggles  often  terminate  in  the 
complete  subordination  of  the  posterior  zooid:  it  is  not 
torn  loose  from  its  attachment,  but  lets  go  its  hold 
and  no  longer  reacts  independently.  Later,  when  the 
anterior  individual  has  become  more  quiet,  the  same 
procedure  may  occur  again.  Evidently  as  the  activity 
of  the  anterior  individual  increases  the  range  of  domi- 
nance increases,  and,  if  fission  does  not  occur  at  once,  the 
posterior  zooid  may  finally  be  brought  under  control. 
Moreover,  one  of  the  simplest  ways  of  inducing  fission 
in  this  species  is  to  cut  off  the  head  of  the  anterior  indi- 
vidual. Such  animals  creep  about  even  in  the  absence 
of  the  head,  but  under  these  conditions  the  posterior 
zooid  is  more  completely  physiologically  isolated  and 
separation  soon  occurs  if  the  tissues  are  not  too  tough.' 
Experiments  to  be  described  below  will  show  other 
ways  in  which  the  existence  of  dominance  can  be  demon- 
strated and  its  range  varied  and  controlled  in  these  and 
other  animals  and  in  many  plants. 

THE   INDEPENDENCE   OF   THE   APICAL  REGION 

The  apical  region  of  the  organic  individual  is,  to  a 
large  extent,  independent  and  is  capable  of  developing, 

'  For  a  more  extended  consideration  of  the  process  of  fission  and  the 
various  indications  of  the  presence  of  the  posterior  zooids  see  Child, 
"Physiological  Isolation  of  Parts  and  Fission  in  Planaria,"  Archiv 
fiir  Entwickelungsmeclmnik,  XXX,  II.  Teil,  1910;  "Studies  on  the 
Dynamics  of  Morphogenesis,  etc.,  Ill,"  Jour,  of  Ex  per.  Zool.,  XI,  191 1; 
"Studies,  etc.,  VI."  Archiv  fiir  Entwickelungsmechanik,  XXXV,    191.^. 


PHYSIOLOGICAL  DOMINANCE  97 

at  least  to  an  advanced  stage,  in  the  complete  absence 
of  other  parts  of  the  body.  This  independence  is  very 
evident  in  Tubularia  and  Planaria.     Pieces  one  or  two 


Fig.  47. — Reconstitution  of  single  and  biaxial  apical  structures  from 
short  pieces  of  stem  of  Tubularia,  to  illustrate  independence  of  apical 
region. 

millimeters  in  length  cut  from  the  stem  of  Tubularia 
usually  develop  into  hydranths  with  a  very  short  stem 
or  partial  hydranths  with  more  or  less  of  the  basal  region 
absent  (Fig.  47).     The  result  depends  on  the  condition 


gS  INDIVIDUALITY  IN  ORGANISMS 

of  the  animal,  the  length  of  the  piece,  and  the  level  of 
the  stem  from  which  it  is  taken.  The  shorter  the  piece 
from  a  given  level  of  the  stem  the  more  completely  is  its 
development  limited  to  apical  parts,  as  Fig.  47  shows. 
The  shortest  pieces  give  rise  to  nothing  but  the  apical 
ends  of  the  hydranths,  with  mouths  and  the  apical  row 
of  tentacles.  In  no  case  do  such  pieces  produce  basal 
parts  of  the  hydranth  without  apical  parts.  Where 
anything  is  missing  it  is  always  the  more  basal  region, 
either  stem  or  more  or  less  of  the  basal  hydranth  region. 
The  results  in  such  pieces  constitute,  I  believe,  a  demon- 
stration that  the  apical  region  of  the  individual  arises 
first  and  other  regions  are  determined  later,  as  far  as  the 
length  of  the  piece  permits. 

The  development  of  hydranths  or  apical  portions  of 
hydranths  may  occur  at  one  or  both  ends  of  such  short 
pieces  as  indicated  in  Fig.  47.  This  difference  arises 
according  as  the  original  metabohc  gradient  in  the  stem 
is  more  or  less  marked.  In  such  short  pieces  of  the 
stem  the  difference  in  metabolic  rate  at  the  two  ends  of 
the  piece  is  but  sHght  in  any  case.  If,  however,  the  rate 
at  the  apical  end  of  the  piece  is  enough  higher  than  that 
at  the  basal  end,  development  at  the  apical  end  pro- 
ceeds more  rapidly  than  at  the  basal  end,  the  apical 
end  is  dominant,  and  the  piece  produces  a  single  hydranth 
or  part.  But  if  the  gradient  is  very  slight  in  the  piece 
the  two  ends  react  at  the  same  rate,  and  since  the 
presence  of  the  wound  at  each  end  brings  about  an 
increase  in  metabolic  rate  at  each  end,  equal  or  nearly 
equal  gradients  in  opposite  directions  arise  and  hydranths 
or  apical  parts  arise  at  both  ends  with  their  axes  opposed. 
Often,  even  in  such  cases,  the  original  gradient  appears 


PHYSIOLOGICAL  DOMINANCE  99 

in  the  smaller  size  or  more  incomplete  condition  of  the 
structure  formed  at  the  basal  end  of  the  piece.  In  Fig. 
47  one  case  near  the  bottom  of  the  figure  is  shown  in 
which  one  end  is  a  hydranth  with  both  sets  of  tentacles, 
the  other  a  partial  hydranth  w^ith  only  the  apical  set  and 
the  reproductive  organs.' 

In  Planaria  the  development  of  short  pieces  is 
essentially  similar.  Short  pieces  from  various  levels  of 
the  body  may  undergo  complete  transformation  into 
single  or  double  heads  without  the  formation  of  other 


4>> 

Fig.  48. — Reconstitution  of  single  and  biaxial  apical  structures  from 
short  pieces  of  Planaria,  to  illustrate  independence  of  apical  region. 

parts  of  the  body  or  with  more  or  less  of  the  anterior  body- 
region  (Fig.  48).  When  a  single  head  arises,  it  is  at  the 
anterior  end  of  the  piece.  The  conditions  determining 
the  development  of  these  heads  are  the  same  as  those  in 
Tuhularia.  As  in  Tubularia  also,  the  original  gradient 
may  appear  to  some  extent  in  the  more  rapid  and  more 
complete  development,  larger  size,  and  dominance  in 
motor  activity  of  the  head  at  the  original  anterior  end 
of  the  piece,  as  in  the  case  at  the  right  of  Fig.  48. 

^  For  more  extended  consideration  see  Child,  "Analysis  of  Form- 
Regulation  in  Tuhularia.  V,  Regulation  in  Short  Pieces,"  Archiv  fiir 
Entwickelungsmechanik,  XXIV,  1907;  "Die  physiologische  Isolation 
von  Teilen  des  Organismus,"  Vortrage  und  Aufsiitze  iihcr  Eiitwickduugs- 
mechanik,  H,  XI,  191 1,  101-19.  Further  references  are  given  in  these 
papers. 


100  INDIVIDUALITY  IN  ORGANISMS 

These  double  apical  regions  and  heads  have  been 
observed  by  many  investigators  in  various  animals  and 
have  commonly  been  called  axial  heteromorphoses, 
because  the  apical  structure  at  the  basal  end  of  the  piece 
was  regarded  as  something  which  was  out  of  place  and 
abnormal.  This,  however,  is  not  actually  the  case,  for 
the  development  of  these  double  or  biaxial  structures  is, 
as  I  have  shown,  subject  to  exactly  the  same  laws  as  the 
development  of  the  usual  single  individual,  only  in  these 
pieces  the  conditions  are  such  that  the  original  gradient 
is  almost  absent,  and  the  increased  activity  at  the  basal 
end  may  establish  a  new  gradient  in  the  reverse  direction, 
although  some  indication  of  the  original  gradient  may 
remain  in  the  smaller  size  or  less  complete  development 
of  the  part  at  the  basal  end.  In  these  short  pieces,  in 
fact,  the  original  polarity  is  almost  obliterated  and  the 
establishment  of  a  new  reversed  polarity  in  relation 
to  the  basal  cut  end  is  possible.  At  each  end  the 
relation  between  the  metabolic  gradient  and  the  devel- 
opment of  an  apical  structure  is  exactly  the  same  as 
in  any  other  case  of  development.  The  apical  region 
arises  at  the  apical  end  of  the  gradient  and  the  devel- 
opment of  other  parts  follows  as  far  as  the  gradient 
extends  from  each  end,  or  in  the  case  of  single  struc- 
tures as  far  as  the  length  of  the  piece  permits.  By 
means  of  the  susceptibility  method  I  have  been  able 
to  demonstrate  these  relations  between  the  metabolic 
gradients  and  the  single  or  double  development  of  such 
pieces. 

The  development  of  biaxial  or  multiple  apical  struc- 
tures from  pieces  has  been  observed  in  various  other 
animals,   and,    while   their   relations   to   the   metabolic 


PHYSIOLOGICAL  DOMINANCE  loi 

gradients  have  not  been  determined,  their  character  and 
the  conditions  of  their  development  indicate  that  when- 
ever an  apical  structure  arises  it  represents  the  a[)ical 
region  of  a  metabolic  gradient. 

In  the  plants  also  conditions  are  apparently  similar. 
The  apical  region  of  a  plant  individual  may  arise  inde- 
pendently of  other  parts,  and  if  it  becomes  structurally 
connected  with  them  later  the  connection  develops 
progressively  from  the  new  apical  region  toward  other 
parts  and  not  in  the  opposite  direction.  The  formati(jn 
of  buds  on  the  leaves  of  begonia  and  in  wound  callus, 
described  above  (pp.  83-86),  are  cases  in  point,  and 
many  other  similar  cases  might  be  cited.  The  develop- 
mental gradients  in  such  cases  indicate  that  the  new 
apical  structure  or  part  represents  the  apical  region  of  a 
metabolic  gradient. 

These  conclusions  concerning  the  independence  of  the 
apical  region  and  its  relation  to  the  metabolic  gradient, 
which  are  based  upon  experimental  demonstration  for 
certain  cases  and  highly  convincing  evidence  for  others, 
are  in  full  agreement  with  the  facts  of  embryonic  develop- 
ment. There  also,  so  far  as  experimental  evidence  has 
been  obtained,  the  apical  region  of  the  individual  is  the 
apical  region  of  a  metabolic  gradient,  and  precedence 
of  the  apical  region  in  development  and  the  develop- 
mental gradients  in  the  direction  of  the  major  iixis 
indicate  that  this  relation  is  general.  I  belie\e 
we  are  justified  in  concluding  that  in  this  respect 
development  of  the  organic  individual  is  always  and 
everywhere  the  same.  Further  evidence  in  support 
of  this  conclusion  will  be  presented  in  the  following 
pages. 


I02  INDIVIDUALITY  IN  ORGANISMS 

DOMINANCE   AND   SUBORDINATION   IN   EXPERIMENTAL 

REPRODUCTION 

The  existence  of  a  relation  of  dominance  and  sub- 
ordination along  the  major  axis  is  shown  by  the  fact 
that,  while  the  apical  region  is  independent,  other  levels 
of  the  body  can  develop  only  in  organic  connection  with 
more  apical  levels  or  with  the  apical  region  itself.  In 
Tubularia  and  related  forms  stolons  arise  only  in  relation 
to  stems  or  hydranths  and  stems,  stem  regions  appear 
only  in  relation  to  higher  levels  in  the  gradient,  etc. 
Stolons  may  grow  out  from  stems  in  the  absence  of 
hydranths,  and  under  certain  conditions  when  the  meta- 
bolic gradient  is  slight  stolons  may  even  arise  at  both 
ends  of  a  piece  of  stem,  but  no  case  has  ever  been 
observed  of  the  development  of  a  stolon  independently  of 
other  more  apical  levels. 

This  relation  is  also  very  evident  in  Planaria.^ 
The  reconstitutional  development  of  pieces  from  the 
middle  and  posterior  regions  of  the  anterior  individual, 
such  as  a  and  b  in  Fig.  49,  ranges  according  to  the 
physiological  condition  of  the  animal  and  with  experi- 
mental conditions  from  a  normal  complete  animal  like 
Fig.  50  through  various  intermediate  forms,  of  which  the 
anophthalmic  is  shown  in  Fig.  51,  to  headless  forms,  like 
Figs.  52  and  53.  The  headless  forms  produce  all  parts 
of  the  body  basal  to  the  level  which  they  represent,  but 
never  give  rise  to  any  part  characteristic  of  more  apical 
levels.  The  reason  why  they  do  not  produce  heads 
will  appear  in  the  following  section.     Thus,  headless 

'  Child,  "Studies  on  the  Dynamics  of  Morphogenesis,  etc.,  I," 
Jour,  of  Ex  per.  ZooL,  X,  1911;  11,  ibid., Xl,  191 1;  "Experimental  Control 
of  Morphogenesis  in  the  Regulation  of  Plavaria,"  Biol.  Bull.,  XX,  igii. 


iPHYSIOLOGICAL  DOMINANCE 


103 


forms  from  the  level  h 
of  Fig.  49  give  rise  to 
new  tails  and  to  all  parts 
below  their  own  level 
(Fig.  53).  but  never 
produce  a  mouth  and 
pharynx,  while  headless 
forms  from  the  level  a 
or  any  level  apical  to  it 
give  rise  to  mouth  and 
pharynx  as  well  as  to 
postpharyngeal  regions 
(Fig.  52),  but  never  to 
regions  representing 
more  apical  levels  than 
themselves. 

If,  however,  a  head 
of  any  sort,  even  a  rudi- 
mentary, anophthalmic 
head,  like  that  of  Fig. 
51,  with  no  eyes  and 
small,  very  incompletely 
developed,  cephalic 
ganglia,  arises  on  a  piece 
from  the  level  h,  then 
the  regions  of  the  piece 
adjoining  the  new  head 
give  rise  to  the  parts 
representing  all  levels 
between  the  head  and 
the  level  which  the  piece 
h  occupied  in  the  original 


U 


a 


^^ 


«<' 


mm 


sS'-L. 


Figs.  49~53- — Phuiar'ui  doroto- 
cephala:  Fig.  49,  outline  indicating 
regions  a  and  h  from  wliich  pieces  arc 
taken;  Figs.  50-53,  dilTercnt  results 
of  reconstitution,  depending  on  pres- 
ence or  absence  of  a  new  head -region. 

individual.     In  other  words, 


T04  INDIVIDUALITY  IN  ORGANISMS 

the  development  of  parts  apical  to  the  original  level  of 
the  piece  takes  place  only  in  relation  to  the  development 
of  a  new  apical  end,  while  the  development  of  parts  basal 
to  the  original  level  of  the  piece  is  determined  by  the 
piece  itself,  even  in  the  absence  of  a  head.  All  the  facts 
indicate  that  the  same  relation  exists  in  other  animals. 
It  has  already  been  pointed  out  (pp.  83-86)  that 
when  new  growing  tips  arise  in  wound  callus  or  from 
differentiated  cells  of  plants,  growth  and  differentiation 
proceed  from  these,  not  toward  them.  Plant  stems, 
lateral  branches,  and  leaves  are  subordinate  parts  or 
individuals  of  the  plant  and  develop  only  under  the 
dominance  of  growing  tips.  The  root  of  the  higher 
plant  is  likewise  a  subordinate  individual.  It  possesses 
a  growing  tip  and  between  this  growing  tip  and  other 
parts  of  the  root  indi\ddual  the  same  relations  of  domi- 
nance and  subordination  exists  as  between  the  stem-tip 
and  other  levels  of  the  stem,  but  the  root  as  a  whole 
develops  only  in  subordination  to  S(mie  part  of  the  plant, 
a  stem-tip,  a  stem,  a  branch,  a  bud,  a  leaf,  or  some  part 
of  a  root  system  already  present.  The  same  is  true  for 
the  root-like  structures,  the  rhizoids  of  the  lower  plants. 
The  roots  and  rhizoids  of  the  plant  have  apparently 
much  the  same  relation  to  the  organism  as  a  whole  as 
do  the  stolons  of  Ttibularia  and  related  fonns.  They 
are  individuals,  each  with  an  axial  gradient  and  a 
dominant  region  of  their  own,  but  they  are  specialized 
individuals,  and  arise  from  the  basal  region  of  the 
major  axis  of  the  individual  which  controls  their  forma- 
tion, whether  it  is  a  single  bud  or  branch,  a  leaf,  or  the 
whole  stem  of  a  composite  plant  individual.  It  is  prob- 
able that  these  subordinate  individuals  really  represent 


PHYSIOLOGICAL  DOMINANCE 


10! 


partially  inhibited  gradients  (see  pp.  178-81).  Certain 
external  conditions,  such  as  moisture  and  darkness, 
favor  the  development  of  roots,  but  do  not  determine 
their  origin.  It  is  commonly  stated  by  botanists  that 
roots  may  arise  on  any  or  almost  any  part  of  a  plant 
where  external  conditions  permit  their  development 
or  where  the  need  for  them  exists.  This  is  true  in  a 
sense,  because  most  plants  are  composite  individuals, 
and  when  one  of  the  constituent  individuals  of  the  plant, 
such  as  a  bud,  branch,  or  leaf,  is  sufficiently  isolated 
from  an  existing  root  system,  or  under  certain  external 
conditions,  that  individual  may  develop  a  root  or  roots. 
Physiologically  or  physically  isolated  parts  of  a  plant  may 
undergo  transformation  into  stem-tips  without  relation 
to  other  parts  and  the  stem-tips  determine  the  fomiation 
of  other  parts,  but  even  though  various  parts  of  plants 
may  give  rise  to  roots  in  the  absence  of  stem-tips,  in  no 
case  does  any  other  isolated  part  of  a  plant  undergo 
transformation  into  roots  alone.  Moreover,  in  develop- 
ment in  nature  roots  and  rhizoids  in  general  arise  only 
after  the  primary  apical  region  has  been  determined. 
They  are,  in  short,  subordinate  to  the  individual  as  a 
whole,  but,  like  leaves  and  various  other  plant  ''organs," 
possess  a  certain  degree  of  individuation  of  their  own. 
The  question  of  the  nature  of  the  correlative  influence  of 
the  root  system  upon  other  parts  of  the  {)kint  is  one  of 
considerable  interest  and  is  touched  upon  in  cluq).  \- 
(pp.  159-63)- 

THE   RECONSTITUTION   OF  AN   INDIVIDUAL   FROM  AN 

ISOLATED   PIECE 

In  the  case  of  Planaria  dorotocephala  it  has  been 
possible  to  analyze  the  process  of  reconstitution  to  some 


io6  INDIVIDUALITY  IN  ORGANISMS 

extent  and  so  to  control  it  experimentally  in  various 
ways,  and  my  experiments  have  led  to  certain  conclusions 
concerning  the  nature  of  reconstitution.  A  part  of  the 
evidence  on  which  these  conclusions  are  based  has 
already  appeared  in  various  papers,'  but  some  of  it  is  still 
unpublished.  Here  only  some  of  the  more  important 
points  and  the  conclusions  are  briefly  presented.  The 
results  of  the  reconstitution  of  pieces  in  Planaria  doro- 
tocephala  differ  widely  in  different  cases.  I  have  found 
it  convenient  to  distinguish  five  different  forms:  the 
normal  (Fig.  50,  p.  103),  an  individual  in  all  respects 
like  the  type  of  the  species;  teratophthalmic  (Fig.  54, 
A,  B),  in  which  the  eyes  show  various  degrees  of  fusion, 
inequality,  or  other  departures  from  the  usual  condition, 
but  the  head  as  a  whole  shows  the  usual  form;  terato- 
morphic  (Fig.  55,  A,  B),  usually  with  a  single  eye  in  the 
median  line  and  the  cephalic  sensory  lobes  more  or  less 
approximated  or  completely  fused  at  the  front  of  the 
head  instead  of  in  a  lateral  position;  anophthalmic 
(Fig.  56,  A,  B),  with  an  outgrowth  more  or  less  like  a 
head  and  containing  a  small  ganglionic  mass,  sometimes 
with  cephahc  lobes  fused  at  the  front,  but  without  eyes; 
headless  (Figs.  52,  83,  p.  103),  in  which  the  cut  end 
merely  heals  without  outgrowth  of  new  tissue.  Differ- 
ent  degrees   of   development    of    the    cephalic   ganglia 

'  Child,  'Studies  on  the  Dynamics  of  Morphogenesis,  etc.,  I," 
Jour,  of  Ex  per.  ZooL,  X,  1911;  II,  ibid.,  XI,  1911;  IV,  ibid.,  XIII,  1912; 
VII,  ibid.,  XVI,  1914;  VIII,  ibid.,  XVII,  1914.  See  also  Child,  "Experi- 
mental Control  of  Morphogenesis  in  the  Regulation  of  Planar^'a" 
Biol.  Bull.,  XX,  191 1 ;  "Certain  Dynamic  Factors  in  Experimental 
Reproduction  and  Their  Significance  for  the  Problems  of  Repro- 
duction and  Development,"  Archiv  fUr  Entimckelungsmechanik,  XXXV. 
1913 


PHYSIOLOGICAL  DOMINANCE 


107 


correspond  to  these  different  types  of  hcad^  and  are 
undoubtedly  the  fundamental  factors  in  determining 
general  form  and  localization  of  the  parts  in  the  head"! 


54^ 


J    ^:- 


CM.: 


^^c 


(H 


fC 


cm 


lu 


y  ', 


n: 


545 


Figs.  54-56. — Different  results  of  reconstitution  in  Planaria 
dorotocephala:  Fig,  54^,  teratophthalmic  animal;  Fig.  54^,  dilTercnt 
forms  of  eyes  in  teratophthalmic  animals;  Fig.  55.4,  B,  teratomoqihic 
forms;  Fig.  56^,  B,  anophthalmia  forms. 

As  a  matter  of  fact,  these  different  forms  are  a  more  or 
less  arbitrary  grouping  of  what  is  actually  a  graded  scries 
of  forms  from  the  normal  head  at  one  extreme  to  the 

^  See  Child  and  McKie,  "The  Central  Nervous  System  in  Tera- 
tophthalmic and  Teratomorphic  Forms  of  Planaria  dorolocep/hila," 
Biol.  Bull.,  XXII,  191 1. 


io8 


INDIVIDUALITY  IN  ORGANISMS 


0  0 


a 


c 


headless  form  at  the  other.  I  have  determined  experi- 
mentally that  these  different  forms 
represent  different  degrees  of  retarda- 
tion or  inhibition  of  the  process  of 
head  formation.  Their  formation  can 
be  controlled  experimentally  in  a 
great  variety  of  ways.  For  example, 
the  percentage  of  pieces  producing 
heads,  which  we  may  call  the  head- 
frequency,  is  less  in  shorter  than  in 
longer  pieces,  in  pieces  from  more 
basal  than  in  those  from  more  apical 
levels  of  the  body ;  less  in  pieces  from 
young  than  in  pieces  from  old  ani- 
mals, in  pieces  from  starved  than  in 
pieces  from  well-fed  animals,  in  pieces 
which  are  kept  quiet  than  in  those 
forced  to  move  about. 

The  effect  on  head-frequency  of 
substances  which  decrease  metabolic 
rate,  such  as  dilute  solutions  of 
cyanides  and  narcotics,  is  of  great 
interest,  for  it  is  definite  and  modi- 
fiable experimentally,  but  not  uni- 
form. In  series  of  pieces  of  equal 
length,  a,  5,  c,  Fig.  57,  taken  from 
animals  of  the  same  size  and  as 
nearly  as  possible  the  same  physio- 
logical condition,  the  head-frequency 
under  natural  conditions  is  highest  in 
the  a-pieces  which  represent  the  most 

apical  region  below  the  head  of  the  anterior  zooid,  in 


Fig.  57. — Outline 
of  Planaria  doroto- 
cephala,  indicating 
regions,  a,  b,  c,  from 
which  pieces  are 
taken. 


PHYSIOLOGICAL  DOAIINAXCE 


109 


y 


the  Z>-pieces  it  is  lower,  and  in  the  c-pieccs  lowest  of 
all.  If  such  a  series  of  pieces  is  placed  for  a  few  hours 
after  cutting  in  a  low  concentration  of  cyanide,  alcohol, 
etc.,  the  head-frequency  in  the  a-pieces  is  considerably 
lower  than  in  water,  that  in  the  6-pieces  slightly  lower 
or  about  the  same  as  in  water,  while  that  of  the  c-pieces 
is  higher  than  in  water.  This  result  is  characteristic, 
but  the  actual  percentages  can  be  altered  by  differences 
in  concentration  of  the  reagents,  tem- 
peratures, and  many  other  factors. 

Although  at  first  glance  these  re- 
sults appear  hopelessly  confusing, 
they  depend  upon  a  very  simple  rela- 
tion between  that  region  of  the  piece 
which  gives  rise  to  the  head  and  other 
parts.  In  an  isolated  piece  of  the 
planarian  body  (Fig.  58)  the  head 
arises  from  the  cells  of  the  region  x, 
which  are  more  directly  affected  by 
the  wound  and  undergo  rapid 
dedifferentiation  and  rejuvenescence 
and  so  attain  a  higher  metabohc  rate  than  cells  farther 
away  from  the  cut  surface  and  begin  soon  after 
section  to  divide  and  grow  rapidly.  If  these  cells  give 
rise  to  a  head,  the  region  y  undergoes  more  or  less 
transformation  to  form  the  body  of  the  new  individual. 
I  have  found  that  the  head-frequency  varies  directly 
with  the  metabohc  rate  in  x,  the  head-forming  region, 
and  inversely  with  the  metabolic  rate  in  the  region  ;•. 
This  relation  may  be  stated  in  the  formula,  head- 
frequency  =  ?^  .     This  means  that  the  higher  the  mcta 


Fig.  58.  —  Dia- 
grammatic outline  of 
a  piece  of  Planaria  to 
illustrate  relations  of 
new  apical  region,  .v, 
new  basal  region, 
:;,  and  old  body 
region,  y. 


rate  y 


bolic  rate  in  x,  the  more  likely  the  piece  is  to  give  rise 


no  INDIVIDUALITY  IN  ORGANISMS 

to  a  head,  and  vice  versa,  and  it  also  means  that  the 
higher  the  metabolic  rate  in  the  region  y,  the  less  likely 
the  piece  is  to  give  rise  to  a  head.  If  this  relation  is 
altered  by  an  increase  of  rate  x  relatively  to  rate  y,  head- 
frequency  is  increased;  if  by  an  increase  in  rate  y  rela- 
tively to  rate  x^  head-frequency  is  decreased.  On  this 
basis  all  the  experimental  effects  of  different  physio- 
logical and  external  conditions  on  head-formation  can 
be  readily  accounted  for,  and  it  has  even  been  possible 
in  many  cases  to  predict  the  results  of  various  experi- 
ments. 

Some  of  the  facts  on  which  this  conclusion  is  based 
are  as  follows:  By  means  of  the  susceptibility  method  I 
have  demonstrated  that  the  act  of  section  always  in- 
creases metabolic  rate,  particularly  in  the  part  basal 
to  the  cut.  This  condition  of  stimulation  continues  in 
the  pieces  for  several  hours  after  cutting  and  only  gradu- 
ally disappears.^  The  more  basal  the  level  of  the  piece 
in  the  original  body,  the  more  its  metabolic  rate  is 
increased  by  section.  In  the  cases  of  pieces  a,  6,  c  in 
Fig.  57  the  metabolic  rate  during  the  first  few  hours 
after  section  is  higher  in  h  than  in  a  and  higher  in  c  than 
in  &,  although  before  section  the  rate  decreased  from 
a  to  c.  This  difference  in  stimulation  of  pieces  from 
different  levels  results  from  the  different  degrees  of 
subordination.  The  region  c  is  subordinate  to  all  more 
apical  regions  and  is  much  more  dependent  upon 
impulses  coming  from  these  regions  than  is  the  region  a, 
which  is  subordinate  only  to  the  head.  When  the  chief 
paths  of  conduction  in  the  nervous  system  are  cut  they 

'  Child,  "Studies  on  the  Dynamics  of  Morphogenesis.  VII," 
Jour,  of  Exper.  ZooL,  XVI,  1914. 


PHYSIOLOGICAL  DOMINANCE  1 1 1 

are  stimulated ;  consequently  the  more  basal  the  levc!  of 
a  piece  the  more  its  rate  is  increased  by  section.  I 
have  also  found  that  the  shorter  a  piece,  the  higher  the 
metabohc  rate  after  section.  Long  pieces  are  stimulated 
but  Httle,  except  at  the  ends,  chiefly  the  apical,  but  in 
short  pieces  the  rate  increases  greatly. 

Another  simple  experiment'  shows  that  under  ordi- 
nary conditions  it  is  determined  within  three  to  six 
hours  after  section  whether  or  not  a  head  will  develop 
on  a  piece.  This  is  during  the  period  of  stimulation  of 
the  piece,  and  when  we  compare  head-frequencies  and 
metabolic  rates  during  the  period  of  stimulation  follow- 
ing section,  we  see  that  the  higher  the  metabolic  rate 
in  the  piece  as  a  whole,  i.e.,  the  region  y,  Fig.  58,  the  less 
likely  a  head  is  to  develop,  and  vice  versa.  The  head- 
frequency  is  lower  in  more  basal  pieces  such  as  c  than 
in  more  apical  pieces  like  a,  and  in  shorter  than  in  longer 
pieces,  because  the  metabolic  rate  in  the  region  y  is  higher 
at  the  time  of  determination  of  the  course  of  develop- 
ment. Pieces  from  young  or  starved  animals  also  have 
a  higher  metabolic  rate^  and  a  lower  head-frequency 
than  similar  pieces  from  old  or  well-fed  animals. 

These  facts  may  seem  to  involve  a  paradox,  but 
their  interpretation  is  actually  simple.  The  two  regions 
X  and  y  (Fig.  58)  of  the  piece  behave  differently  after 
section.  The  cells  of  x  are  so  extremely  affected  by 
the  presence  of  the  wound  and  the  altered  conditions 
that  they  rapidly  dedifferentiate  and  begin  to  divide 
and   grow,   and   so   approach   or  attain   an   embryonic 

'  Child,  "Studies  on  the  Dynamics  of  Morphogenesis.  VIII," 
ibid.,  XVII,  1914. 

2  Child,  Senescence  and  Rejuvenescence,  1915,  pp.  i55~63- 


112  INDIVIDUALITY  IN  ORGANISMS 

condition.  The  cells  of  y^  however,  merely  undergo  a 
temporary  increase  in  metabolic  rate.  The  region  a:  is  a 
small  groups  of  cells  undergoing  dedifferentiation,  while  y 
represents  a  considerable  portion  of  a  fully  developed 
individual  with  established  relations  of  parts  and  spe- 
cialized nerves  which  are  much  more  efficient  than 
embr^^onic  protoplasm  as  conducting  paths.  The  region 
X  originally  has  a  higher  metabolic  rate  than  y,  because 
it  represents  a  more  apical  level  in  the  gradient,  and  its 
rate  rises  still  farther  as  it  begins  to  dedifferentiate. 
The  facts  of  experiment  indicate  that  in  order  to  produce 
a  new  head  rate  x  must  not  merely  be  higher  but  much 
higher  that  rate  y.  The  relation  between  x  and  y 
is  evidently  this:  if  rate  x  is  sufficiently  above  rate  y,  x 
develops  independently  of  y  into  a  head  and  dominates 
y,  while  otherwise  y  dominates  a:  to  a  greater  or  less  ex- 
tent and  so  retards  or  inhibits  head-formation,  and  the 
various  forms  between  normal  and  headless  condition 
are  produced. 

This  relation  ^^^  can  be  altered  in  various  way-s: 
by  means  of  dilute  narcotics  it  is  possible  according  to 
the  method  of  use  either  to  decrease  the  stimulation 
in  y  resulting  from  section  or  to  delay  the  reaction  in  x 
until  after  the  increased  rate  in  y  has  largely  or  wholly 
disappeared,  or  finally  the  relation  may  be  altered  by  the 
more  rapid  and  more  complete  acclimation  of  the  young 
cells  at  X  as  compared  with  the  older  cells  of  y  (see 
pp.  51,  52).  In  pieces  such  as  c.  Fig.  57,  where  under 
ordinary  conditions  x  and  y  are,  so  to  speak,  evenly 
matched  in  the  struggle  for  dominance  and  the  head- 
frequency  is  low,  all  these  methods  increase  the  head- 
frequency,  because  the  relative  increase  in  rate  of  x  as 


PHYSIOLOGICAL  DOMINANCE  113 

compared  with  y  overbalances  the  absolute  decrease  in 
rate  produced  by  the  narcotic.  In  pieces  like  a,  where  y 
is  only  slightly  stimulated  by  section  and  rate  x  is  so 
much  higher  than  rate  y  that  the  head-frequency  is 
very  high,  the  effect  of  narcotics  is  to  decrease  head- 
frequency,  because  in  such  cases  dominance  is  not 
reversed  and  only  the  direct  inhibiting  effect  of  the 
narcotic  on  the  region  x  appears. 

Head-frequency  may  be  increased  in  all  pieces  by 
inducing  them  to  move  about.'  The  apical  end,  of 
course,  precedes  in  such  movement,  the  cells  of  the  region 
X  are  subjected  to  more  excitation  than  in  a  piece  which 
is  not  moving,  and  the  higher  metabolic  rate  of  x  results 
in  increased  head-frequency. 

In  Planaria  maculata  and  certain  other  species  the 
degree  of  subordination  of  basal  regions  of  the  body  is  not 
as  great  as  in  P.  dorotocephala;  consequently  the  increase 
in  metabolic  rate  after  section  in  pieces  from  this  region  is 
less  than  in  P.  dorotocephala,  and  in  these  species  the 
head-frequency  of  such  pieces  is  almost  or  quite  as  great 
as  that  in  more  apical  pieces.  Various  other  differences 
in  the  reconstitutional  process  in  different  species  of 
planarians  only  serve  to  confirm  the  conclusions  reached 
in  the  case  of  P.  dorotocephala. 

These  and  many  other  facts  have  forced  me  to  the 
conclusion  that  the  head  wliich  appears  in  the  recon- 
stitution  of  a  piece  is  not  physiologically  a  part  of  the 
piece  and  is  not  formed  by  the  piece,  but  develops,  so  to 
speak,  in  spite  of  it.  Only  when  the  metabolic  rate 
of  the  cells  at  x  is  high  enough  to  make  them  essentially 

'  Child,  "Experimental  Control  of  Morphogenesis  in  the  Regulation 
of  Planaria,^'  Biol.  Bull.,  XX,  191 1. 


114  INDIVIDUALITY  IN  ORGANISMS 

independent  of  y  do  they  begin  the  development  of  a 
new  individual  by  the  formation  of  a  head.  The 
formation  of  a  head  at  the  end  of  a  piece  is  then  exactly 
the  same  process  as  the  transformation  of  short  pieces 
into  heads  when  no  other  part  of  the  body  is  present 
(pp.  96-101).  The  new  head  arises  independently  of 
other  parts  and  dominates  them.  The  influence  of 
other  parts  on  head-formation  is  merely  inhibitory  or 
negative,  while  the  influence  of  head -formation  on  other 
parts  is  determinative  or  positive.  The  process  of 
development  of  the  cephalic  ganglia  in  the  head  formed 
on  a  piece  also  indicates  the  independence  of  the  head. 
The  ganglia  arise  in  the  new  tissue  independently  of  the 
parts  of  the  nervous  system  in  the  old  tissue  of  the  piece 
and  become  connected  with  these  parts  only  secondarily.' 
This  fact  suggests  that  head-formation  actually  depends 
upon  the  establishment  of  a  melabolic  gradient  in  the 
region  x  with  its  apical  region  near  the  free  end  and 
decreasing  in  rate  toward  y.  If  this  occurs,  a  head  forms, 
but  if  rate  y  is  high  enough  in  relation  to  rate  x,  head- 
formation  is  inhibited  or  retarded  by  the  interference 
between  two  gradients  in  opposite  directions.  Inhibition 
or  retardation  of  head-formation  consists  then  in  the 
interference  of  one  metabolic  gradient  with  another  in 
the  opposite  direction  or  the  obliteration  of  the  one  by 
the  other. 

The  new  basal  end  of  the  piece  develops  from  a  grou[) 
of  cells  2,  Fig.  58,  which  react  to  the  wound  at  the  basal 
end  by  more  or  less  dedifferentiation  and  growth.     This 

'  S.  Flexner,  "The  Regeneration  of  the  Nervous  System  of  Pianaria 
lorva,"  etc.,  Jour,  of  MorphoL,  XIV,  1898;  Child  and  McKie,  "The 
Centjal  Nervous  System  in  Teratophthalmic  and  Teratomorphic  Forms 
of  Pianaria  dorotocephala,'"  Biol.  Bull.,  XXII,  191 1. 


PHYSIOLOGICAL  DOMINANCE  115 

reaction  is  less  rapid  than  that  of  x  (see  Fig.  31),  because 
they  represent  a  lower  level  in  the  gradient  and  their 
relation  to  the  region  y  is  different  from  that  of  x.  The 
rate  of  development  and  completeness  of  the  new  basal 
end  varies  directly  with  the  metabolic  rate  in  y;  any 
conditions  which  decrease  the  metabolic  rate  in  v 
decrease  the  development  of  the  basal  end,  and  vice 
versa.  We  may  say  then  that  tail-frequency  =  "]-^-^  but 
that  under  the  usual  conditions,  when  a  gradient  is 
already  present  in  the  piece,  rate  z  is  so  low  that  it 
becomes  negligible,  and  the  formula  becomes  tail- 
frequency  =  rate  y.  This  holds  true  as  long  as  a  new 
zooid  does  not  arise  in  this  basal  region.  If  a  new  zooid 
does  arise  there  in  consequence  of  physiological  isolation, 
as  is  often  the  case  in  headless  pieces,  then  the  lower 
the  rate  in  y,  the  more  rapid  the  development  of  this 
posterior  zooid.  In  headless  pieces  the  large  size  of  the 
posterior  outgrowth  (cf.  Figs.  52,  53,  with  Fig.  50,  p.  103) 
is  due  to  the  fact  that  this  region  is  not  physiologically 
the  basal  end  of  the  individual  but  a  second  individual.' 
The  development  of  the  basal  region  is  then  depend- 
ent upon  the  presence  and  influence  of  more  apical 
regions,  while  the  development  of  the  head  occurs 
independently  of  other  parts,  so  far  as  it  is  not  inhibited 
by  them.  The  relation  between  the  major  axial  gradient 
and  these  differences  of  behavior  in  different  regions  is 
evident.  The  process  of  reconstitution  of  a  new  indi- 
vidual from  a  headless  piece  in  Planaria  is  a  process  of 
development  beginning  at  two  different  levels,  first,  at 
the  apical  end  of  the  piece  with  the  formation  of  a 

^  Child,  "Studies  on  the  Dynamics  of  Morphogenesis.     TIT,"  Jour, 
of  Exper.  Zool.,  XI,  igii. 


Ii6  INDIVIDUALITY  IN  ORGANISMS 

new  head,  and,  secondly,  at  the  basal  end  with  the 
formation  of  a  new  tail.  The  new  apical  region  as  the 
region  of  highest  metabolic  rate  determines  the  estab- 
lishment of  a  new  major  axial  gradient,  which  has  the 
same  direction  as  the  original  gradient  but  possesses  a 
higher  rate,  and  in  consequence  of  these  changes  the 
parts  of  the  piece  below  the  new  apical  region  undergo 
more  or  less  structural  change  into  parts  characteristic 
of  more  apical  levels,  until  sooner  or  later  a  stable  con- 
dition of  the  gradient  is  attained,  and  this  determines 
the  completion  of  reconstruction. 

It  can  scarcely  be  doubted  that  the  process  of  recon- 
stitution  of  pieces  into  new  individuals  is  fundamentally 
the  same  in  all  animals,  though  it  may  differ  widely  in 
details,  with  the  kind  and  physiological  condition  of  the 
individual  or  piece  and  the  nature  of  the  external  con- 
ditions under  which  reconstitution  occurs.  Moreover, 
it  is  essentially  the  same  process  as  reconstitution  in 
plants,  in  that  it  consists  in  the  development  of  a  new 
individual  beginning  with  the  apical  end.  The  chief 
difference  is  that  in  animals  the  development  of  the  new 
individual  is  usually  closely  associated  with  the  cut 
surface  or  surfaces,  while  in  plants  the  reaction  of  the  cells 
at  the  cut  surface  usually  does  not  at  once  cover  it  with 
more  or  less  embryonic  rapidly  growing  cells,  as  it  does  in 
animals,  and,  since  the  plant  is  usually  a  composite 
individual,  other  apical  regions  already  present  become 
dominant,  or  new  apical  regions  arise  in  other  parts  before 
a  new  apical  region  develops  at  the  cut  surface.  In 
some  cases,  where  only  a  small  part  of  the  apical  region  is 
removed,  a  new  growing  tip  develops  from  the  cut 
surface,  and  in  such  cases  the  formation  of  the  new  grow- 


PHYSIOLOGICAL  DOMINANCE  1 1 7 

ing  tip  is,  I  believe,  essentially  the  same  process  as  the 
formation  of  the  new  head  in  Planaria.  In  cases  where 
wound  callus  develops,  new  growing  tips  may  arise  in 
that.  In  the  formation  of  a  new  growing  tip  in  callus 
tissues  (pp.  85-86)  and  its  later  connection  with  other 
parts  of  the  plant  we  have  again  a  process  very  simihir 
to  the  formation  of  a  head  in  a  piece  of  Planaria,  and  the 
development  under  its  dominance  of  other  parts,  so  far 
as  they  are  not  already  present.  In  both  cases  the  new 
apical  region  is  not  determined  by  other  parts  but 
develops  independently  of  them,  and  its  later  relations 
to  them  are  determined  by  its  own  dominance. 

SOME   MODIFYING   AND   LIMITING   FACTORS   IN  ANIMAL 

RECONSTITUTION 

The  development  of  double  or  biaxial  apical  regions 
from  short  pieces  has  been  discussed  above  (pp.  98,  99). 
In  some  cases  biaxial  basal  regions 
instead  of  apical  regions  arise  from 
pieces.  Pieces  of  the  stems  of  certain 
hydroids  sometimes  produce  stolons 
at  both  ends,  biaxial  tails  have  been  p^^  59  — Experi- 
observed  in  short  pieces  of  Planaria  mentally  determined 
by  Morgan,  and  I  have  been  able  to     reconstitution  of 

-^  o      7  ^  1,      .  biaxial  basal  ends  in 

produce  them  experimentally  m  some  ^.^^^  ^^  pianaria. 
cases  (Fig.  59)  by  altering  the  relations 
of  metabolic  rate  between  the  regions  x  and  y  (Fig.  58) 
with  the  aid  of  narcotics.  In  the  earthworm  and  related 
forms  various  investigators  have  observed  the  develop- 
ment of  tails  at  both  ends  of  pieces  from  the  more  basal 
regions  of  the  body.  My  own  experiments  indicate  that 
when  the  development  of  the  new  tissue  at  a  cut  end  of 


ii8  INDIVIDUALITY  IN  ORGANISMS 

a  piece  is  completely  dominated  by  the  piece  it  gives 
rise  to  a  basal  structure.  Such  dominance  means  simply 
that  the  old  tissue  has  a  high  enough  metabolic  rate  to 
determine  the  direction  of  the  gradient  in  the  new  tissue. 
In  Planaria  the  development  of  tails  at  both  ends  of  a 
short  piece  is  apparently  due  simply  to  the  fact  that  the 
metabolic  rate  in  the  piece  is  high  enough  so  that  the 
new  tissue  does  not  become  dominant  at  either  end  but 
develops  under  the  control  of  the  old  tissue.  Dr.  Hyman 
has  found  that  the  conditions  determining  the  formation 
of  double  tails  in  Lumhriculus  seem  to  be  essentially  the 
same  as  in  Planaria,  though  the  factors  which  produce 
them  are  somewhat  different.  She  has  been  able  to 
determine  experimentally  to  some  extent  the  produc- 
tion of  heads  instead  of  tails  in  such  pieces  by  methods 
similar  to  those  which  I  have  employed  for  altering 
head-frequency  in  Planaria.  She  has  also  observed  the 
development  of  structures  intermediate  between  head 
and  tail,  or  rather  inhibited,  rudimentary  cephalic  ends, 
in  which  certain  caudal  characteristics  appear  later. 
These  are  apparently  cases  in  which  the  new  tissue  was 
at  first  to  some  extent  independent  but  later  became 
subordinated  to  the  old.^ 

The  absence  of  any  outgrowth  at  the  apical  end  of  a 
piece,  as  in  the  headless  forms  of  Planaria  (Figs.  52,  53), 
occurs  when  head-formation  is  completely  inhibited, 
but  the  degree  of  dominance  is  not  sufficient  to  deter- 
mine development  as  a  tail.  In  such  cases  local  con- 
ditions at  the  cut  apparently  determine  the  result  and 
the  wound  simply  heals.     In  some  other  forms  the  wound 

^  I  am  indebted  to  Dr.  Hyman  for  permission  to  use  these  unpub- 
lished data. 


PHYSIOLOGICAL  DOMINANCE  119 

reaction  involves  more  growth  than  in  Planaria,  and  in 
such  cases  considerable  outgrowths  may  sometimes  arise 
which  are  neither  heads  nor  tails,  but  cell  masses  of 
indeterminate  character  which  gradually  differentiate 
in  relation  to  adjoining  parts  and  may  finally  show  both 
apical  and  basal  characteristics. 

In  many  of  the  flatworms  and  various  other  forms 
only  an  apical  cut  surface  above  a  certain  level  of  the 
body  gives  rise  to  a  head,  while  tails  may  arise  from  cut 
surfaces  at  any  level  basal  to  the  head  of  the  parent 
body.  In  some  of  these  cases  the  level  where  head- 
formation  ceases  lies  a  considerable  distance  from  .the 
cephalic  ganglia,  while  in  other  cases  head-formation 
does  not  occur  when  the  cephalic  ganglia  are  removed; 
but  when  parts  of  the  head  are  removed  leaving  a  por- 
tion of  the  cephalic  ganglia  intact — sometimes  half  or 
more,  sometimes  only  a  small  part,  is  necessary — such 
parts  develop  again.  In  the  headless  pieces  there  may  be 
more  or  less  outgrowth  at  the  apical  end  of  the  piece, 
but  it  is  indeterminate  in  character.  Some  authors 
have  maintained  that  in  such  cases  the  cephalic  ganglia 
or  the  more  apical  regions  of  the  longitudinal  nerve 
cords  exercise  a  specific  formative  influence  of  some 
sort  and  so  determine  the  development  of  a  new  head, 
but  there  is  no  real  evidence  in  favor  of  this  \'icw. 
Probably  the  head  fails  to  develop  in  such  cases  either 
because  the  cells  reacting  to  the  wound  do  not  attain 
a  high  enough  metabolic  rate  to  become  independent 
of  other  parts  and  their  development  into  a  head  is 
therefore  inhibited,  as  in  the  headless  pieces  of  Planar ia. 
or  because  these  cells  do  not  dedifferentiate  to  a  suffi- 
cient extent  to  be  capable  of  giving  rise  to  a  new  cephalic 


120  INDIVIDUALITY  IN  ORGANISMS 

ganglion  and  so  to  a  head,  while  they  may  still  be  able 
under  the  dominance  of  other  parts  to  produce  basal 
regions  of  the  body.  The  development  of  the  apical 
region  or  of  the  apical  part  of  the  central  nervous  system, 
which  in  all  except  the  lowest  animals  is  the  primary  and 
dominant  part  of  the  apical  region,  is  a  self-determining 
process,  independent  of  other  parts,  while  the  develop- 
ment of  other  parts  is  determined  by  their  relations 
to  dominant  regions.  It  is  highly  probable  therefore 
that  a  more  complete  loss  of  differentiation  is  necessary 
as  a  condition  for  head-formation  than  for  the  develop- 
ment of  other  parts.  As  a  matter  of  fact,  we  find  that 
as  the  capacity  for  reconstitution  becomes  limited  by 
increasing  differentiation  the  capacity  for  head-formation 
disappears  first  of  all.  Many  animals  in  which  recon- 
stitution of  new  heads  does  not  occur  are  still  able  to 
reproduce  all  subordinate  parts,  and  with  further  limita- 
tion it  is  the  more  subordinate  parts,  such  as  legs  and 
other  appendages  or  caudal  regions,  which  the  body  is 
capable  of  reproducing. 

This  limitation  is  more  or  less  progressive  from  lower 
to  higher  forms,  until  in  the  higher  vertebrates  the 
capacity  for  reconstitution  under  any  known  con- 
ditions is  limited  practically  to  tissue  regeneration. 
The  primary  limiting  factor  is  unquestionably  the 
increasing  physiological  stability  of  the  protoplasmic 
substratum,  in  consequence  of  which  the  capacity  for 
dedifferentiation  and  rejuvenescence,  at  least  under 
ordinary  conditions,  is  more  and  more  narrowly 
limited.^ 

^  Child,  Senescence  and  Rejuvenescence,  1Q15,  pp.  35,  39,  41-43,  53, 
194,  267,  304,  463-65.  : 


PHYSIOLOGICAL  DOMINANCE 


121 


The  embryonic  stages  of  different  animals  differ 
widely  as  regards  their  capacity  for  reconstitution. 
In  the  sea-urchin  and  starfish  isolated  cells  or  groups 
ol  cells  of  the  developing  embryo  down  to  a  cer- 
tain limit  may  give  rise  to  complete  larvae  of  small 
size,  while  in  other  forms,  such  as  the  annelids  and 
mollusks,  isolated  parts  of  the  embryo  show  little  or 
no  reconstitutional  change,  but  remain  alive  for  a  time 
and  continue  to  differentiate  as  they  do  when  they 
remain  as  parts  of  an  intact  embryo.  From  the  failure 
of  the  isolated  parts  to  undergo  reconstitution  the  con- 
clusion has  been  drawn  that  they  are  independent  of 
each  other  in  the  intact  embryo,  and  that  development 
-in  these  organisms  is  a  sort  of  mosaic  made  up  of  inde- 
pendent parts  with  some  sort  of  pre-established  harmony 
between  them.  If  this  view  is  correct,  there  is  no  rela- 
tion of  dominance  and  subordination  in  these  stages 
of  development.  The  failure  of  isolated  parts  to  undergo 
reconstitution  does  not,  however,  demonstrate  the  ab- 
sence of  dominance  but  merely  the  ineffectiyeness 
of  isolation.  The  absence  or  limitation  of  embryonic 
reconstitution  in  certain  forms  is  apparently  due,  like 
the  increasing  limitation  of  reconstitutional  capacity 
in  higher  animals,  to  the  higher  specialization  of  the  parts 
of  the  egg  and  embryo  in  these  forms.  There  is  good 
reason  to  believe  that  in  such  eggs  the  condition  in  em- 
bryonic stages  is  the  result  of  differentiation  dependent 
upon  dominance  and  subordination  of  parts  in  the  earlier 
life  of  the  egg,  and  that  specialization  has  gone  beyond 
the  stage  where  it  can  be  greatly  altered  by  isolation. 
Development  proceeds  in  isolated  parts  as  far  as  it  has 
been  determined  by  past  relations  with  other  parts  or  as 


122  INDIVIDUALITY  IN  ORGANISMS 

far  as  nutritive  or  other  conditions  permit,  and  then 
ceases.  There  can  be  Uttle  doubt  that  relations  of 
dominance  and  subordination  exist  during  embryonic 
stages,  and  that  these  are  factors  in  determining  what 
occurs  in  later  stages.  According  to  this  view,  the 
difference  between  these  eggs  and  those  in  which  a  high 
degree  of  embryonic  reconstitution  occurs  is  primarily 
a  difference  in  the  stability  or  fixity  of  the  effects  of 
previously  established  metabolic  gradients.  At  the  one 
extreme  are  eggs  in  which  axial  differences  at  the  be- 
ginning of  embryonic  development  are  probably  largely 
or  wholly  differences  in  metabolic  rate,  at  the  other, 
those  in  which  specialization  and  differentiation  of  parts 
have  gone  far  beyond  this  condition.  The  egg,  in  short, 
is  an  individual,  and  some  eggs  are  more  highly  special- 
ized individuals  than  others. 

The  proportional  relations  of  parts  in  reconstitution, 
of  which  much  has  been  made  by  Driesch,  Morgan,  and 
others,  are  obviously,  so  far  as  they  exist,  dependent 
upon  metabolic  relations  between  the  parts.  On  a  short 
piece  of  Planaria,  for  example,  a  smaller  head  usually 
develops  than  on  a  long  piece.  This  fact  has  often  been 
regarded  as  in  some  way  associated  with  the  fact  that 
the  shorter  piece  will  produce  a  smaller  animal  than  the 
longer  and  that  the  size  of  the  new  head  foreshadows 
the  size  of  the  animal.  As  a  matter  of  fact,  the  size 
of  the  head  formed  by  pieces  of  the  same  size  may 
differ  widely  in  different  cases  and  can  be  controlled 
experimentally  to  a  very  large  extent  by  controUing 
metabolic  conditions.  The  higher  the  metabolic  rate  in 
the  region  x,  Fig.  58,  in  relation  to  that  of  the  region  y, 
the  larger  the  head,  and  vice  versa.     The  size  of  tJie 


PHYSIOLOGICAL  DOMINANCE  123 

head  in  relation  to  other  parts  is  determined  primarily 
by  its  ability  to  grow  at  their  expense.  In  a  shorter 
piece  there  is  less  material  available  for  such  growth 
than  in  a  longer  piece,  consequently  a  smaller  head 
develops.  Essentially  the  same  relation  exists  as 
regards  other  parts.  Where  an  excess  of  nutritive 
material  is  available  the  relation  is  not  necessarily  very 
different,  for  each  part  uses  nutrition  instead  of  the 
substance  of  other  parts  according  to  its  metabolic 
activity,  i.e.,  according  to  its  position  in  the  axial 
gradients,  so  that  in  this  case  also  the  chief  factors  in 
determining  the  proportions  of  parts  characteristic  of 
each  form  are  the  metabolic  relations  between  them. 
In  the  early  stages  of  development  in  nature  the  simple 
quantitative  gradation  in  size  from  the  apical  toward 
the  basal  region  appears,  but  as  specialization  occurs 
and  the  differences  in  metabolic  rate  at  different  levels 
bring  about  changes  in  metabolic  character  the  size  rela- 
tions must  of  course  become  more  complex. 

The  return  or  approach  to  the  characteristic  form  of 
the  species  which  very  commonly  takes  place  in  the 
reconstitution  of  pieces  has  been  regarded  by  JMorgan 
and  others  as  largely  a  matter  of  the  physical  rearrange- 
ment of  the  substance  of  the  piece.  That  changes  in 
shape  may  be  brought  about  in  soft-bodied  forms  like 
the  flatworms  by  mechanical  conditions  connected 
with  motor  and  other  functional  activities  of  the  ani- 
mals,  I  have  shown.'     Wherever  such  factors  play  a 

^  Child,  "Studies  on  Regulation.  IV,"  Jour.  ofExpcr.  Zodl..  I.  1QO.4: 
VII,  ibid.,  II,  1905;  "Studies  on  Regulation.  LX,  X,"  Arch,  jtir 
Entwickelungsmechanik,  XX,  1905;  "The  Regulatory  Change  of  Shape 
in  Planaria  dorotocephala,'"  Biol.  Bull.,  XVI,  1909. 


124  INDIVIDUALITY  IN  ORGANISMS 

part  in  determining  the  characteristic  shape  of  the 
animal  they  undoubtedly  play  a  part  in  determining 
the  approach  to  this  shape  in  pieces  undergoing  recon- 
stitution,  but  in  cases  where  they  are  not  primarily  con- 
cerned the  metabolic  relations  are  unquestionably  the 
primary  factors  in  determining  shape  and  proportions 
of  the  whole  and  parts. 

In  most  adult  animals  and  embryonic  stages  which 
are  capable  of  any  considerable  degree  of  reconstitutional 
reproduction,  a  limit  of  size  of  isolated  pieces  seems  to 
exist  below  which  reconstitution  becomes  incomplete  or 
fails  to  occur.  In  Planaria,  for  example,  with  decrease 
in  size  of  piece  head-frequency  falls  to  zero,  but  with  still 
further  decrease  in  size  head-formation  begins  to  occur 
again  and  head-frequency  rises.  These  changes  are 
simply  due  to  changes  in  the  relation  JJ^  (see  pp.  109- 
10).  With  decreasing  size  of  the  piece,  y  is  more  and 
more  highly  stimulated  by  section  until  in  pieces  below 
a  certain  size  heads  do  not  develop  at  all,  but  when 
the  piece  becomes  very  small  y  practically  disappears, 
for  the  whole  piece  becomes  involved  in  the  direct 
wound  reaction  and  so  corresponds  to  the  region  x 
or  such  a  region  in  relation  to  both  cut  ends.  In  such 
pieces  there  is  nothing  to  inhibit  or  retard  head- 
formation  except  the  simultaneous  development  of  a 
head  at  the  opposite  end  (see  pp.  98-101),  and  in 
such  cases  the  effect  is  mutual  and  results  merely  in 
retardation. 

Here  then  the  completeness  or  incompleteness  of 
reconstitution  in  relation  to  size  of  piece  is  wholly  a 
matter  of  quantitative  metabolic  relations.  There  is 
no  minimal  size  of  piece  which  represents  the  '^organi- 


PHYSIOLOGICAL  DOMINANCE  1 2 :: 

zation"  of  the  species  reduced  to  its  lowest  terms.  The 
minimal  size  can  be  altered  widely  even  now  by  con- 
trolling conditions,  and  I  have  no  doubt  that  if  we  arc 
ever  able  to  isolate  single  cells  and  to  provide  proper 
nutritive  and  other  conditions  for  them  we  shall  fmd  that 
in  many  of  the  lower  animals  such  cells  are  capable  of 
giving  rise  to  new  individuals,  as  they  undoubtedly  are 
in  many  plants. 

Most  investigators  have  regarded  the  minimal  size 
of  pieces  undergoing  reconstitution  as  something  abso- 
lute and  have  failed  entirely  to  note  that  it  differs  with 
the  physiological  condition  of  the  animal,  the  region  of 
the  body,  and  the  various  external  conditions  which 
affect  metabolic  rate.  To  determine  the  smallest  piece 
of  animal  capable  of  reconstitution  under  given  con- 
ditions is  merely  to  determine  one  special  case  out  of  an 
indefinite  number  of  possible  cases. 

CONCLUSION 

The  experimental  evidence  demonstrates,  first,  the 
essential  independence  of  the  apical  region  in  both 
plants  and  animals,  and,  secondly,  determination  and 
control  by  this  apical  region  of  the  developmental 
processes  at  other  levels  of  the  major  axis  of  the  indi- 
vidual. The  reconstitution  of  pieces  into  new  individ- 
uals is  fundamentally  the  same  process  as  embryonic 
development,  and  the  same  relation  of  dominance  and 
subordination  exists  in  both.  The  different  results  of 
reconstitution  in  pieces  of  different  size,  from  differ- 
ent levels,  in  different  physiological  conditions,  under 
different  environmental  conditions,  etc.,  depend  pri- 
marily upon  relations  of  dominance  and  subordination, 


126  INDIVIDUALITY  IN  ORGANISMS 

determined  by  the  relations  of  metabolic  rate  in 
different  parts.  In  the  higher  animals  other  factors, 
such  as  the  stability  of  the  differentiated  cellular 
substratum,  may  contribute  to  limit  reconstitutional 
capacity. 


CHAPTER  V 

THE  RANGE  OF  DOMINANCE,  PHYSIOLOGICAL 

ISOLATION,  AND  EXPERIMENTAL 

REPRODUCTION 

If  the  conception  of  physiological  dominance  which 
is  presented  in  chap,  ii  is  correct,  the  existence  of  a 
transmission-decrement  in  the  impulses,  stimuli,  or  excita- 
tions which  are  the  effective  agents  in  dominance  must 
determine  a  certain  range  of  dominance  and  therefore 
a  physiological  size  limit  or  limit  of  length  for  each  axis, 
which  cannot  be  exceeded  without  physiological  isola- 
tion of  the  part  that  lies  beyond  the  range  of  domi- 
nance. Moreover,  the  limit  of  dominance  in  a  given  case 
must  vary  with  the  metabolic  rate  in  the  dominant 
region  and  the  conductivity  along  the  path  of  trans- 
mission. Its  effectiveness  upon  a  subordinate  part  ma\' 
also  depend  upon  the  receptivity  of  the  part  to  the 
transmitted  excitations,  and  this  may  be  determined 
by  local  conditions  to  which  the  part  is  subjected.  If 
the  characteristic  gradients  are  present  or  arise  in  a 
physiologically  isolated  part,  such  a  part  may  become 
a  new  complete  individual,  if  it  is  not  so  highly  special- 
ized or  differentiated  as  to  be  incapable  of  reacting  to  the 
altered  conditions  by  dedifferentiation  and  redevelop- 
ment. Some  of  the  evidence  bearing  upon  these  aspects 
of  the  problem  of  dominance  is  considered  in  this 
chapter, 

127 


128 


INDIVIDUALITY  IN  ORGANISMS 


EXPERIMENTAL   CONTROL   OF   SPATIAL  RELATIONS   OF 
PARTS   AND   OF   THE  RANGE   OF  DOMINANCE 

The  dimensions  and  distance  relations  of  parts  along 
an  axis  can  be  altered  by  altering  the  metabolic  rate  in 
the  dominant  region  or  throughout  the  organism  and 
so  increasing  or  decreasing  the  length  of  the  gradient. 


i   f 


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61 


Figs.  60-62. — Different  lengths  of  hydranth  primordium  in  recon- 
stitution  of  pieces  of  Tuhularia:  Fig.  60,  length  at  medium  metabolic 
rate;  a,  b,  c,  d,  the  four  regions  of  the  primordium;  Fig.  61,  length  at 
high  metabolic  rate;  Fig.  62,  length  at  low  metabolic  rate. 

In  Tuhularia  the  reconstitutional  development  of  a 
hydranth  is  a  transformation  inside  the  perisarc  of  the 
terminal  region  of  the  piece  into  a  hydranth  without 
outgrowth  of  new  tissue  from  the  cut  surface.  In  the 
early  stages  of  this  process  (Figs.  60-62)  the  two  rows 
of  tentacles  arise  as  two  series  of  longitudinal  ridges 
(Fig.  60,  h,  d),  usually  distinguishable  from  other  parts 


THE  RANGE  OF  DOMINANCE  129 

by  accumulations  of  red  pigment.  Various  facts,  some 
of  which  have  been  mentioned  above  (pp.  79,  96--99), 
show  that  the  parts  of  the  hydranth  are  determined 
from  the  apical  end  in  the  basal  direction.  The  point 
of  present  interest  in  this  process  is  the  lenirth  of  stem 
concerned  in  the  formation  of  the  new  hydranth  and 
the  length  of  each  of  the  four  distinguishable  regions, 
a,  b,  c,  d,  of  the  developing  hydranth.  In  pieces  ol  like 
physiological  condition  kept  under  the  same  external 
environment  these  lengths  show  a  high  degree  of  con- 
stancy, but  they  can  readily  be  altered  by  altering  the 
metaboHc  rate  in  the  pieces.  Fig.  60  shows  the  length 
and  proportions  of  the  early  stage  of  a  hydranth  develop- 
ing with  a  medium  metabolic  rate,  Fig.  61,  with  a  high 
rate,  and  Fig.  62,  with  a  very  low  rate.  Evidently  the 
higher  the  metabolic  rate  the  greater  the  distance  from 
the  end  of  the  stem  and  from  each  other  at  which  the 
two  rows  of  tentacles  arise.  The  relative  lengths  of 
the  different  parts  also  change  with  metabolic  rate, 
that  of  the  region  a  increasing  and  that  of  the  region 
d  decreasing  with  increasing  metabolic  rate,  and  vice 
versa.^ 

The  position,  size,  and  time  of  appearance  of  hy- 
dranths  and  the  relation  of  hydranths  to  other  parts  in 
the  reconstitution  of  isolated  pieces  of  Tubular  in  and 
related  forms  have  been  repeatedly  investigated,  but, 
although  the  facts  are  very  definite,  the  various  authors 

^  Child,  "An  Analysis  of  Form  Regulation  in  Tubularia.  II, 
Differences  in  Proportion  in  the  Primordia,"  Archiv  fur  Enhvickelungs- 
mechanik,  XXIII,  1907.  In  this  paper  I  showed  that  such  dilTcrcnccs  in 
proportion  appeared  in  hydranths  from  different  levels  and  ends  of  the 
stem,  but  it  is  now  known  that  these  differences  in  level  really  represent 
differences  in  metabolic  rate. 


I30  INDIVIDUALITY  IN  ORGANISMS 

have  failed  to  reach  any  very  satisfactory  general  inter- 
pretation of  them.  Driesch,  who  has  used  Tubularia  to 
a  large  extent  as  experimental  material,  even  maintains 
that  they  cannot  be  interpreted  on  a  physico-chemical 
basis.  As  a  matter  of  fact,  however,  not  only  do  the 
facts  fall  readily  into  line  with  the  dynamic  conception  of 
the  individual  which  I  have  outlined,  but  many  of  them 
constitute  valuable  evidence  for  that  conception. 

I  have  found  that  previously  existing  metabolic 
gradients  in  the  stem  of  Tubularia  are  rapidly  obliterated 
and  new  gradients  readily  arise  when  metabolic  con- 
ditions change.  This  is  due  to  the  fact  that  the  proto- 
plasmic substratum  is  not  very  stable,  and,  except  in 
the  hydranth,  there  is  little  structural  differentiation 
in  relation  to  the  metabolic  gradient.  Wherever  the 
stem  of  Tubularia  is  cut  across,  and  even  in  many  cases 
where  section  is  not  complete,  a  metabolic  gradient 
arises  in  connection  with  the  stimulation  of  the  wound 
and  the  open  end  exposed  to  sea-water  and  the  oxygen 
contained  in  it.  The  region  of  highest  rate  in  this 
gradient  is  at  the  cut  end,  and  the  gradient  extends  a 
greater  or  less  distance  from  the  cut,  according  to  the 
physiological  condition  of  the  stem  and  the  direction  and 
metabolic  rate  of  the  pre-existing  gradient  in  the  region 
concerned.  If  the  metabolic  gradient  resulting  from 
stimulation  at  the  cut  end  is  in  the  same  direction  as  the 
pre-existing  gradient,  then  of  course  there  is  merely  an 
augmentation  of  the  gradient,  but  if  two  gradients  are 
in  opposite  directions,  as  they  are  at  the  basal  end  of 
a  piece,  they  tend  to  neutralize,  obliterate,  or  inhibit 
each  other,  and  the  one  which  has  the  higher  metabolic 
rate  sooner  or  later  obliterates   the  other.     The  evi- 


THE  RANGE  OF  DOMINANCE  131 

dence  indicates  that  when  such  a  gradient  is  sufficiently 
marked,  that  is  to  say^  when  the  metaboHc  rate  in  its 
apical  region  is  sufficiently  high,  and  when  the  inhibiting 
or  obliterating  influence  of  a  gradient  in  the  oi)posite 
direction  is  not  too  great,  a  hydranth  develops.  The 
formation  of  a  stolon,  on  the  other  hand,  apparently 
represents  a  gradient  which  is  partially  inhibited  or 
obliterated,  or,  in  other  words,  partially  dominated  by  a 
gradient  in  the  opposite  direction,  but  in  addition  to  this 
relation  a  relatively  high  metabolic  rate  in  the  piece 
or  individual  as  a  whole  is  also  apparently  necessary 
for  stolon-formation.  The  stem  represents  the  lower 
levels  of  a  simple  uninhibited  gradient,  and  its  formation 
always  occurs  under  the  dominance  of  a  hydranth  or 
other  region  of  higher  metabolic  rate. 

It  is  also  important  for  an  understanding  of  the  facts 
to  note  that  in  general  the  metabolic  rate  of  these  animals 
decreases  when  they  are  transferred  from  natural  to 
laboratory  conditions,  and  the  hydranths  which  develop 
in  the  laboratory  possess  a  lower  metabolic  rate  than 
those  in  nature;  consequently  the  range  of  dominance 
is  less  and  physiological  isolation  occurs  at  shorter 
distances  from  the  dominant  regions  than  in  animals  in 
nature.  Moreover,  the  development  of  a  new  hydranth 
at  the  cut  end  of  a  piece  of  stem  is,  I  believe,  a  process 
essentially  similar  to  the  development  of  a  head  on  a 
piece  of  Planaria  (pp.  105-14).  The  new  hydranth 
region  is  independent  of  other  parts  and  becomes 
dominant  over  them,  but  during  the  early  stages  of  its 
development  this  dominance  is  less  complete,  because 
the  changes  in  the  protoplasm  of  the  stem  in  accordance 
with  the  new  metaboHc  conditions  require  some  lime; 


132  INDIVIDUALITY  IN  ORGANISMS 

therefore  removal  of  the  original  hydranth  favors  physio- 
logical isolation  of  basal  regions  of  the  piece. 

In  Corymorpha  the  metabolic  relations  and  the  rela- 
tions of  the  various  parts  of  the  body  to  the  metabolic 
gradients  are  essentially  the  same  as  in  Tuhidaria,  and 
the  demonstration  of  the  metabolic  gradients  by  means 
of  the  susceptibility  method  in  Corymorpha,  where  most 
of  the  stem  is  naked,  is  not  open  to  the  objection  which 
might  be  raised  in  the  case  of  Tubularia,  where  all  parts 
of  the  stem  except  the  cut  end  are  covered  by  the  horny 
perisarc,  viz.,  that  the  reagent  penetrates  the  tissues  only 
or  chiefly  from  the  cut  end  and  so  produces  a  death 
gradient  which  is  merely  a  gradient  of  penetration  and 
does  not  represent  metabolic  conditions. 

Some  of  the  facts  and  their  interpretations  in  terms 
of  metabolic  gradients  and  physiological  dominance 
are  briefly  as  follows:'  In  pieces  of  Tubularia  stem 
eight  or  ten  millimeters  or  more  in  length  and  with  a 
cut  surface  at  each  end  reconstitution  usually  results 
first  in  the  development  of  a  hydranth  at  the  apical  end 
of  the  piece  and  later  of  a  second  smaller  hydranth  at  the 
basal  end  (Fig.  63).  Occasionally  pieces  from  vigorous 
animals  which  evidently  possess  a  high  metabolic 
rate  produce  an  apical  hydranth  and  a  stolon  at  the 
basal  end   (Fig.   64),   but  before  it  attains  any  great 

'  I  have  described  and  discussed  these  facts  in  the  following  papers: 
Child,  "An  Analysis  of  Form  Regulation  in  Tubularia.  I,"  Archiv  fur 
Entunckelungsmechanik,  XXIII,  1907;  IV  and  V,  ibid.,  XXIV,  1907; 
"Die  physiologische  Isolation  von  Teilen  des  Organismus,"  Vortrdge 
und  Aufsdtze  iiber  Entunckelungsmechanik,  H,  XI,  191 1,  96-119.  The 
discovery  since  these  papers  were  written  of  the  existence  of  metabolic 
gradients  and  their  relation  to  physiological  dominance  affords  a  definite 
basis  for  most  of  the  earlier  conclusions  and  interpretations. 


THE  RANGE  OF  DOMINANCE 


^33 


length  this  stolon  gives  rise  to  a  hydranth  at  its  tip. 
This  is  a  process  of  reproduction  like  that  occurrincr  i,^ 
nature  (Fig.  43,  p.  90),  and  differs  from  it  only  in  tliat 
the  distance  of  the  second  hydranth 
from  the  first  is  less  in  the  pieces 
than  in  the  animal  under  natural 
conditions.  This  difference  indi- 
cates that,  as  might  be  expected, 
the  range  of  dominance  of  the 
apical  region  is  less  in  the  experi- 
mental piece  than  in  the  whole 
animal  in  nature. 

In  most  pieces,  however,  the 
dominance  of  the  apical  region  is 
insufficient  to  inhibit  the  establish- 
ment of  a  well-marked  new  gradient 
in  relation  to  the  cut  basal  end  of 
the  piece,  and  so  the  formation  of 
a  hydranth  usually  occurs  at  this 
end  also,  as  in  Fig.  63.  The 
development  of  this  hydranth  is 
usually  delayed,  as  compared  with 
that  of  the  apical  hydranth,  be- 
cause the  establishment  of  the  new 
gradient  is  more  or  less  retarded 
by  the  gradient  already  existing  in 
the  original  direction,  and  the 
shorter  the  piece  the  greater  the 
delay,  because  in  shdrter  pieces  the  dominance  of  the 
apical  region  is  more  complete,  or,  in  other  words,  the 
gradient  from  the  apical  region  is  more  marked  at  the 
basal  end  and  therefore  inhibits  or  retards  to  a  greater 


64 


Figs.  6;^,  64. — Recon- 
stitution  of  longer  pieces 
of  Tuhularia:  Fig.  63, 
usual  result  of  recon- 
stitution  with  hydranth 
at  basal  end;  Fig.  64, 
reconstitution  with 
stolon     at     basal     end. 


134  INDIVIDUALITY  IN  ORGANISMS 

extent  than  in  longer  pieces  the  estabhshment  of  a  new 
gradient  in  the  opposite  direction.  In  pieces  more  than 
eight  or  ten  millimeters  long,  however,  the  local  condi- 
tions at  the  basal  end  usually  determine  the  result 
sooner  or  later,  and  the  new  gradient  is  estabUshed  and  a 
hydranth  develops  here. 

In  pieces  between  eight  or  ten  and  two  or  three  milli- 
meters in  length  neither  hydranth  nor  any  other  out- 
growth arises  at  the  basal  end  in  most  cases.  In  these 
shorter  pieces  the  dominance  of  the  apical  region  is 
sufficient  to  inhibit  the  new  gradient  at  the  basal  end 
to  a  sufficient  degree  to  prevent  hydranth  formation, 
and  the  general  metabolic  rate  in  these  as  in  most 
other  experimental  pieces  is  not  high  enough  for  stolon- 
formation  to  occur. 

In  the  very  short  pieces  described  in  chap,  iv 
(pp.  96-99)  the  difference  in  metabolic  rate  between 
the  two  ends  of  the  piece  dependent  upon  the  original 
gradient  is  so  slight  that  in  many  cases  the  local  condi- 
tions at  the  two  ends  become  the  determining  factors, 
and  hydranths  begin  to  form  simultaneously  or  nearly 
so  at  both  ends,  the  portion  of  each  hydranth  formed 
depending  on  the  length  of  the  piece.  If  the  original 
gradient  in  the  piece  is  sufficient  to  determine  the  more 
rapid  reaction  at  the  apical  end  this  becomes  dominant 
and  a  single,  instead  of  a  double,  structure  arises. 

These  are  the  chief  facts  of  reconstitution  in  Tubu- 
laria  under  ordinary  conditions  and  their  interpretation 
in  terms  of  metabolic  gradients  and  dominance.  It  is 
possible,  however,  to  obtain  more  positive  evidence  in 
support  of  these  interpretations  by  controlling  and 
altering  the  experimental  conditions.     By  diluting  the 


THE  RANGE  OF  DOMINANCE  17^^ 

sea-water  to  a  certain  extent  the  metabolic  rate  in  pieces 
is  increased,  and  under  these  conditions  pieces  which 
in  normal  sea-water  produce  only  hydranths  at  their 
basal  as  well  as  apical  ends  produce  in  a  lar^^e  percentage 
of  the  cases  stolons  which  later  develop  hydranths  at  thrir 
tips.'  The  hydranths  in  such  pieces  are  longer  and 
larger  than  in  normal  sea-water. 

When  a  piece  is  cut  with  a  fully  developed  active 
hydranth  at  its  apical  end,  no  hydranth  appears  at  the 
basal  end  until  the  metaboKc  rate  of  the  apical  hydrantli 
decreases  or  its  death  occurs,  which  in  Tubularia  is 
usually  within  a  few  days  at  most.  In  Corymiorpha 
relations  are  similar.  Evidently,  then,  a  full-grown, 
active,  apical  hydranth  inhibits  the  development  of  a 
basal  hydranth  in  a  piece,  but  a  hydranth  beginning  to 
develop  at  the  apical  end  is  usually  only  able  to  retard 
to  some  extent  the  development  of  the  basal  hydranth. 
The  dominance  of  the  full-grown  hydranth  is  more 
effective  than  that  of  the  early  stages  of  hydranth 
development. 

Various  investigators  have  observed  that  when  the 
development  of  the  hydranth  at  the  apical  end  of  a  piece 
is  inhibited  by  inclosing  this  end  in  paraffin  or  sticking 
it  in  the  sand  the  development  of  the  hydranth  at  the 
basal  end  is  accelerated,  and  it  has  been  found  that  in 
such  cases  the  basal  hydranth  is  longer  and  larger  than 
when  the  apical  hydranth  is  not  inhibited.  Evident l\- 
the  inhibition  of  development  at  the  apical  end  decreases 
dominance  and  the  estabhshment  of  the  new  gradient, 
and  so  the  development  of  a  hydranth  at  the  basal  inM\ 

'Child,  "An  Analysis  of  Form  Regulation  in  Tubularia.  1.' 
Archiv  fur  Entwickelungsmechanik,  XXIII,  1907 


136  INDIVIDUALITY  IN  ORGANISMS 

is  accelerated.  The  same  result  may  be  attained  by 
compressing,  sharply  bending,  or  partially  crushing  the 
stem  at  some  point  between  the  two  ends.  In  such  cases 
the  influence  of  the  dominant  apical  region  is  prevented 
from  reaching  the  basal  end,  which  is  therefore  physio- 
logically isolated  and  the  establishment  of  the  new 
gradient  but  little  retarded.  Often  also  the  develop- 
ment of  the  basal  hydranth  can  be  accelerated  by  cutting 
partly  through  the  stem,  so  that  only  a  slender  organic 
connection  between  the  two  ends  remains.  In  these  and 
various  other  ways  the  controlling  influence  of  the  apical 
region  can  be  demonstrated. 

Neither  the  inhibition  of  development  of  the  basal 
hydranth  by  paraffining  the  basal  end  or  sticking  it  in 
sand  nor  the  partial  crushing  or  bending  of  the  stem  at  a 
certain  level  influences  the  development  at  the  apical 
end  except  in  very  short  pieces.  In  these,  inhibition  of 
either  end  may  accelerate  the  development  of  the  other, 
and  a  single  instead  of  a  double  structure  may  result. 
These  experiments  show  that  in  the  longer  pieces 
dominance  extends  chiefly  in  the  direction  of  the  original 
gradient,  and  we  find  correspondingly  that  the  new 
gradient  which  arises  at  the  basal  end  does  not  extend 
very  far  from  that  end.  If,  however,  inhibition  of  the 
apical  end  be  continued  for  a  longer  time,  the  gradient 
at  the  basal  end  extends  farther  from  that  end. 

The  length  of  the  hydranths  formed  in  very  short 
pieces  is  often,  though  not  always,  less  than  in  longer 
pieces,  particularly  in  pieces  from  the  more  basal  regions 
of  the  stem.  Driesch  has  made  much  of  this  point  as 
an  indication  that  an  adaptation  of  the  length  of  the 
hydranth  to  the  length  of  the  piece  takes  place  in  order 


THE  RANGE  OF  DOMINANCE  137 

that  a  stem  as  well  as  a  hydranth  may  be  formed. 
According  to  Driesch  this  adaptation  is  not  determined 
physico-chemically,  but  by  the  princi]:)le  which  he 
calls  entelechy  and  which  as  he  believes  controls  develop- 
ment. Unfortunately  for  Driesch's  view  this  "adapta- 
tion" does  not  occur  in  all  cases,  and  is  very  incomplete, 
for,  as  I  have  pointed  out  (pp.  96-99),'  these  short 
pieces  often  give  rise  to  hydranths  or  apical  jjarts  of 
hydranths  without  stems  or  basal  parts.  The  experi- 
mental evidence  indicates  that  the  shorter  hydranths  in 
short  pieces  are  merely  hydranths  which  are  partialis- 
inhibited  by  other  regions  of  the  piece,  just  as  the  head  of 
Planaria  may  be  partially  inhibited  by  other  regions  of 
the  piece.  As  in  Planaria,  short  pieces,  particularly  those 
from  the  more  basal  regions  of  the  body,  are  more  stimu- 
lated by  section,  and  their  metabolic  rate  is  therefore 
higher  throughout  than  that  of  longer  or  more  apical 
pieces.  Under  these  conditions  the  gradient  arising  at 
the  cut  end  is  much  less  effective  in  detennining  the  devel- 
opment of  a  new  structure,  the  hydranth,  than  it  is  when 
the  general  metabolic  rate  is  lower.  Figuratively  speak- 
ing the  new  gradient  is  partially  obliterated  by  the  gen- 
eral high  metabolic  rate  in  the  piece.  Consequently  its 
length  is  less  and  the  length  of  the  hydranth  determined 
by  it  is  correspondingly  less  than  in  longer  pieces,  and 
development  is  also  retarded.  A  piece  of  given  length 
may  produce  a  single  short  hydranth  and  stem,  or  a 
longer  hydranth  without  stem,  or  biaxial  hydranths,  or 
apical  portions,  and  all  these  differences  in  behavior  are 
determined  by  simple  differences  in  the  gradient  relations. 

I  See  also  Child,  "An  Analysis  of  Form  Regulation  in  rubuhiria. 
Regulation  in  Short  Pieces,"  Archiv  fur  Entunckduugsmcchaiiik,  XX I  \". 
1907. 


138  INDIVIDUALITY  IN  ORGANISMS 

In  Planaria  also  the  positions  and  space  relations 
of  parts  along  an  axis  and  the  range  of  dominance 
can  be  altered  and  controlled  by  means  of  conditions 
which  alter  metabolic  rate/  At  ordinary  room  tempera- 
tures in  well-aerated  water  the  isolated  postpharyngeal 
region  of  Planaria  (Fig.  65)  forms  a  new  individual  Uke 
that  in  Fig.  66.  The  new  mouth  and  pharynx  form 
near  the  middle  of  the  piece  at  a  certain  distance  from 
the  new  head,  and  the  region  in  front  of  the  pharynx 
undergoes  the  internal  changes  which  make  it  over  into 
the  prepharyngeal  region  of  the  new  individual.  If,  how- 
ever, the  rate  of  metabolism  in  such  a  piece  is  decreased 
by  means  of  dilute  narcotics,  by  the  presence  of  carbon 
dioxide  and  metabolic  products  in  the  water,  or  by  other 
means,  the  head  develops  slowly,  is  small  and  usually 
abnormal,  and  the  lower  the  metabolic  rate  during 
development  the  nearer  to  the  head  the  mouth  and 
pharynx  arise  and  the  less  the  length  of  the  new  pharyn- 
geal region.  Fig.  67  shows  the  effect  of  a  slight  decrease, 
Fig.  68  of  a  greater,  and  Fig.  69  of  a  still  greater  decrease 
in  metabolic  rate  during  reconstitution.  The  length 
of  the  region  undergoing  reconstitutional  change  is  less 
in  Fig.  67  than  in  Fig.  66,  still  less  in  Fig.  68,  and  in 
Fig.  69  practically  no  changes  occur  below  the  level  of 
the  very  rudimentary  head. 

Reconstitution  of  similar  pieces  with  a  very  high 
metabolic  rate  (at  high  temperature)  results  in  forms 
like  Fig.  70,  in  which  the  pharynx  and  mouth  arise  at  a 

'  Child,  "Physiological  Isolation  of  Parts  and  Fission  in  Planaria," 
Archiv  fur  Entwickelungsmechanik,  XXX  (Festband  fiir  Roux),  II.  Teil, 
1910;  "Studies  on  the  Dynamics  of  Morphogenesis,  etc.  Ill,"  Jour, 
of  ExPer.  ZooL,  XI,  iqii. 


THE  RANGE  OF  DOMINANCE 


139 


greater  distance  from  the  head  and  the  prepharyngeal 
region  is  longer  than  in  Fig.  66. 


\  I 


65 


Figs,  65-70. — Space  relations  of  parts  in  reconstitution  of  Planaria 
dorotocephala  under  different  metabolic  conditions:  Fig.  65,  outline 
indicating  level  of  section;  Fig.  66,  reconstitution  under  standard 
laboratory  conditions;  Figs,  67-69,  different  ranges  of  dominance  and 
space  relations  of  new  parts  in  reconstitution  with  low  metabolic  rate 
in  different  concentrations  of  naroctics;  Fig.  70,  reconstitution  with 
high  metabolic  rate  at  high  temperature. 


I40  INDIVIDUALITY  IN  ORGANISMS 

The  metabolic  gradient  associated  with  the  new 
head  shows  a  corresponding  decrease  and  increase  in 
length  in  such  pieces.  The  influence  of  the  new  head- 
region  extends  to  a  greater  or  less  distance  according  as 
its  metabolic  rate  is  high  or  low,  and  the  position  of  the 
various  organs  is  altered  correspondingly,  or,  as  in  the 
extreme  case  of  Fig.  69,  no  new  organs  are  formed  except 
the  head. 

When  the  metabolic  rate  is  high,  as  in  Figs.  66  and  70, 
dominance  extends  nearly  or  quite  to  the  basal  end  of 
the  piece,  though  short  zooids  may  be  present  as  more 
or  less  distinct  gradients  (see  pp.  92-94)  at  the  basal 
end.  Before  section  most  of  this  region  of  the  body 
consisted  of  one  or  more  zooids,  but  the  development  of  a 
head  nearer  to  these  zooids  than  the  original  head  has 
brought  about  the  obliteration  of  the  gradients  which 
represented  them,  except  perhaps  in  the  extreme  basal 
region,  and  after  reconstitution  a  single  gradient  extends 
over  at  least  most  of  the  length  of  the  piece.  When  the 
metabolic  rate  is  lower,  as  in  Figs.  67  and  68,  a  short 
individual  develops  from  the  apical  region  of  the  piece, 
but  most  of  the  broader  portion  is  not  physiologically  a 
part  of  this  individual.  This  is  very  evident  in  the 
behavior  of  these  forms,  for,  when  creeping  about,  they 
are  unable  to  control  and  co-ordinate  this  region  to  any 
great  extent,  and  simply  drag  it  about  like  a  dead  mass. 
As  long  as  they  remain  in  the  narcotic  they  are  not  active 
enough  to  undergo  fission,  but  if  they  are  returned  to 
water,  fission  may  occur  after  a  few  days,  although  the 
range  of  dominance  gradually  extends,  and  more  and 
more  of  the  length  of  the  piece  comes  under  the  control  of 
the  head. 


THE  RANGE  OF  DOMINANCE  141 

The  different  types  of  head  in  Planaria  (see  pp.  106- 
14)  represent,  as  I  have  pointed  out,  different  degrees 
of  inhibition  of  head-formation,  and,  even  after  develop- 
ment is  completed,  possess  different  metaboHc  rates, 
as  susceptibility  determinations  show.  The  metal joHc 
rate  is  highest  in  the  normal  head,  slightly  lower  in  the 
teratophthalmic,  and  still  lower  in  the  teratomoq^hic  and 
anophthalmic  forms.  In  connection  with  these  differ- 
ences in  the  heads  it  is  of  interest  to  note  that  when  the 
different  forms  are  fed  and  grow,  the  length  which  they 
attain  before  fission  varies  in  general  with  the  form  and 
metabolic  rate  of  the  head.  Under  ordinary  conditions 
normal  animals  usually  become  twelve  or  fifteen  milli- 
meters long  before  undergoing  fission,  teratophthalmic 
forms  usually  slightly  less,  teratomorphic  forms  from 
eight  to  ten  millimeters,  anophthalmic,  from  sLx  to  eight 
or  less,  according  to  the  degree  of  development  of 
the  head-region,  while  headless  forms  rarely  become 
more  than  five  or  six  millimeters  long  before  dividing 
and  often  divide  at  a  length  of  only  three  or  four 
millimeters.  These  differences  indicate  very  clearly 
the  difference  in  range  of  dominance  associated  with 
the  differences  in  metabolic  rate  in  the  dominant 
region. 

There  are  many  ways  of  inducing  advance  in  develop- 
ment of  the  basal  zooids  and  the  occurrence  of  fission  in 
Planaria,  of  which  the  simplest  is  the  removal  of  the 
head  of  the  anunal.  This  decreases  the  degree  and  range 
of  dominance  to  such  an  extent  that  fission  almost 
invariably  occurs  within  a  few  days.  By  removal  of  new 
heads  as  fast  as  they  develop  fission  may  be  induced  even 
in    anunals    much    shorter    than    those    which    usually 


142  INDIVIDUALITY  IN  ORGANISMS 

undergo  fission.'  These  and  various  other  methods  all 
serve  merely  to  increase  the  degree  of  physiological 
isolation  of  the  basal  region  by  decreasing  the  degree 
and  range  of  dominance. 

EXPERIMENTAL    OBLITERATION    AND    DETERMINATION    OF 
AXIAL   GRADIENTS    AND   DOMINANCE 

In  the  case  of  the  hydroid  Corymorpha  (see  pp.  92,  132) 
the  original  gradient  can  readily  be  obliterated  and  the 
estabhshment  of  new  gradients  determined  by  experi- 
mental conditions.  Reconstitution  in  pieces  four  or 
five  millimeters  or  more  in  length  from  the  naked  region 
of  the  stem  in  sea-water  under  the  usual  laboratory  con- 
ditions is  like  that  in  most  of  the  longer  pieces  of  Tuhu- 
laria  stem  (see  Fig.  63,  p.  133).  A  hydranth  develops 
at  the  apical  end  of  the  piece,  and  later  a  second  smaller 
hydranth  appears  at  the  basal  end.  The  metabolic 
conditions  are  also  similar  to  those  in  Tubularia,  and 
reconstitution  can  be  altered  and  controlled  in  much  the 
same  way  in  both  forms.  If,  however,  such  pieces  of 
Corymorpha  are  placed  after  cutting  in  2-2^  per  cent 
alcohol  in  sea-water  the  cut  ends  heal,  but  hydranths  do 
not  develop.  In  the  course  of  a  few  days  the  pieces 
become  shorter  and  more  rounded,  decrease  in  size,  and 
lose  the  characteristic  structure  of  the  Corymorpha 
stem.  The  changes  in  shape  are  indicated  in  Figs.  71 
and  72.  On  removal  to  water  after  several  days  in 
alcohol  a  new  hydranth  begins  to  develop  on  the  upper 
side  of  the  piece  (Fig.  73),  then  a  stem  arises  below  it,  and 

'  Child,  "Physiological  Isolation  of  Parts  and  Fission  in  Planaria,'" 
Archivfiir  Enpwickelungsmechanik,  XXX  (Festband  fiir  Roux),  11.  Teil, 
191Q, 


THE  RANGE  OF  DOMINANCE 


H3 


basal  structures  develop  on  the  lower  side  of  the  piece  in 
contact  with  the  underlying  surface,  and  gradually  the 
piece  is  transformed  into  a  new  small  individual  (Fig.  74). 
In  most  cases  the  old  outline  of  the  piece  is  still  pre- 
served by  a  thin  layer  of  hardened  slime  secreted  by  the 
piece  while  in  alcohol.  This  is  indicated  by  the  dotted 
line    in    Fig.    74.     Susceptibility   determinations    show 


71 


72 


73 


Figs.  71-74. — Experimental  establishment  of  a  new  major  axis  in  a 
piece  of  Corymorpha:  Fig.  71,  the  piece  after  section;  Fig.  72,  after 
reduction  in  alcohol;  Fig.  73,  appearance  of  new  hydranth  on  upper 
side  after  return  to  water;  Fig.  74,  fully  developed  new  individual; 
dotted  lines  indicate  old  outline  of  piece  preserved  by  slime. 

that  in  alcohol  the  original  axial  gradient  disappears, 
and  that  when  the  pieces  are  returned  to  water  a  new 
gradient  arises  in  the  direction  in  which  the  new  axis 
develops.  Since  the  pieces  adhere  to  the  surface  soon 
after  being  placed  in  alcohol,  it  is  possible  to  keep  them 
in  the  same  position  throughout  the  experiment  and  so 
to  be  certain  of  the  original  direction  of  the  major  axis 
and  gradient,  even  though  they  become  hemispherical 


144  INDIVIDUALITY  IN  ORGANISMS 

or  nearly  spherical  in  form.  In  most  cases,  however, 
there  is  no  difficulty  as  regards  this  point,  because  the 
longest  diameter  of  the  pieces  coincides  in  direction  with 
the  original  axis.  A  comparison  of  the  direction  of  the 
new  axis  wliich  arises  after  return  to  water  with  that  of 
the  original  axis  shows  that  the  former  is  at  right  angles 
with  the  latter.  The  new  hydranth  develops  without 
relation  to  either  of  the  cut  ends  from  the  uppermost 
region  of  the  piece  a,s  it  lies  in  the  aquarium,  and  this 
region  was  originally  its  lateral  surface.  In  these  cases 
the  alcohol  not  only  inhibits  the  increase  in  metabolic 
rate  in  relation  to  the  terminal  cut  surfaces,  which 
determines  the  development  of  hydranths  at  the  two 
ends,  but  decreases  the  rate  throughout  the  piece.  In 
this  way  it  obliterates  the  original  gradient  and  domi- 
nance to  such  a  degree  that  when  the  metabolic  rate 
rises  again  on  return  to  water  the  original  axial  relations 
do  not  reappear,  but  a  new  gradient  and  a  new  dominance 
arise  in  relation  to  the  external  conditions  to  which  the 
piece  is  subjected,  and  the  axis  of  the  new  individual 
coincides  in  direction  with  the  new  gradient.  In  all 
cases,  so  far  as  my  experiments  go,  the  new  hydranth 
arises  from  the  uppermost  part  of  the  piece,  no  matter 
what  region  of  the  piece  in  its  original  condition  this 
part  represents. 

When  short  pieces,  which  have  already  produced 
biaxial  hydranths  (Fig.  75),  are  used  for  this  experi- 
ment, the  changes  are  very  similar  to  those  described 
for  longer  pieces.  In  alcohol  the  tentacles  and  the 
apical  regions  of  the  two  hydranths  die  and  disintegrate, 
but  the  more  basal  portions  gradually  lose  their  hydranth 
structure  and  the  pieces  become  small  rounded  masses 


THE  RANGE  OF  DOIMINANCE  145 

in  which  no  structure  is  externally  visible  (Fig.  76). 
After  return  to  water  a  new  hydranth  arises,  as  in  the 
longer  pieces,  on  the  uppermost  part  (Fig.  77),  which 
represents  one  side  of  the  basal  region  of  the  previously 
existing  hydranths,  and  the  piece  undergoes  transforma- 
tion into  a  new  small  individual  (Fig.  78).  In  this 
case  the  two  opposed  metabolic  gradients  which  were 
present  at  the  beginning  of  the  experiment  were 
completely  obliterated  and  a  new  single  gradient 
arises  at  right  angles,   or,  if  the  pieces  are  not  kei)t 


76 

78 


^i: 


iL 


Figs.  75-7S. — Experimental  establishment  of  a  new  major  axis 
in  a  piece  of  Corymorpha  which  has  already  formed  a  biaxial  structure: 
Fig.  75,  the  biaxial  hydranths  developed  from  the  piece;  Fig.  76,  the 
same  piece  after  reduction  in  alcohol;  Fig.  77,  appearance  of  new 
hydranth  after  return  to  watef ;  Fig.  78,  fully  developed  new  individual. 

in  the  same  position  throughout,  in  any  relation  to 
the  original  gradients  as  determined  by  the  external 
conditions. 

My  experiments  along  this  line  were  interrupted 
and  no  opportunity  to  continue  them  has  as  yet  arisen. 
I  beheve,  however,  that  the  new  metabolic  gradient  in 
these  pieces  is  primarily  determined  by  the  diilerencc  in 
oxygen  supply  between  the  free  upper  surface  and  the 
surface  in  contact,  the  region  of  highest  rate  represent- 
ing the  region  of  greatest  oxygen  supply;  but  further 
experiment  is  necessary  to  determine  positively  whetlier 


146  INDIVIDUALITY  IN  ORGANISMS 

this  or  some  other  factor  in  the  environmental  con- 
ditions is  the  essential  one.  The  important  point  is 
that  a  new  metabolic  gradient,  major  axis,  or  polarity 
is  in  these  cases  determined  by  external  conditions, 
and  that  morphogenesis  occurs  with  reference  to  this 
gradient. 

In  the  case  of  a  sea-anemone,  Harenactis  (Fig.  79), 
obliteration  of  the  original  gradient  is  accomplished  in  a 
somewhat  different  way.^  The  bodies  of  these  animals 
are  tubular,  with  partial  longitudinal  partitions,  the 
mesenteries.  When  the  rather  bulky  mesenteries  are 
not  removed,  pieces  cut  from  the  body  close  by  gradual 
contraction  at  each  end,  the  wounds  heal,  and  a  new 
disk  and  tentacles  develop  at  the  apical,  and  a  new 
''foot"  at  the  basal  end.  If,  however,  rather  short 
pieces  are  taken  (a,  h,  Fig.  79)  and  the  mesenteries  are 
largely  cut  away  from  the  interior  of  the  body,  the 
pieces  close  up  and  heal  as  indicated  in  the  longitudinal 
section  (Fig.  80),  because  there  is  no  mass  of  internal 
tissue  to  prevent  the  two  ends  meeting  when  the  piece 
contracts.  In  such  pieces  the  apical  cut  surface  of  the 
body  wall  unites  with  the  basal  about  the  whole  circum- 
ference, and  the  result  is  a  ring  or  doughnut-shaped 
structure  which  makes  an  attempt  to  orient  its  body  as  it 
does  in  nature  by  revolving  about  a  circular  axis  like  a 
vortex  ring  until  the  region  of  union  of  the  two  ends 
lies  on  its  upper  or  outer  surface. 

At  this  region  of  union  more  or  less  new  tissue  arises, 
particularly  if  the  cut  surfaces  are  irregular  and  do  not 

'  Child,  "Factors  of  Form  Regulation  in  Harenactis  attenuata,  I,  II, 
III,"  Jour,  of  Expcr.  ZooL,  VI,  VII,  1909;  "Further  Experiments  on 
Adventitious  Reproduction  and  Polarity  in  Harenactis,"  Biol.  Bull.,  XX, 
1910. 


THE  RANGE  OF  DOMINANCE 


M7 


Figs.  79-83. — Reconstitution  in  "rings"  from  sea-anemone, 
Harenactis  attemiata:  Fig.  79,  longitudinal  sectional  outline  of  animal, 
indicating  regions,  a,  b,  from  which  pieces  are  taken;  Fig.  80,  diagram- 
matic longitudinal  section  through  a  ''ring,"  showing  method  of  closure 
by  union  of  apical  and  basal  cut  surfaces  of  body  wall;  I-'igs.  81,  82, 
tentacle  groups  arising  from  the  region  of  union  of  cut  surfaces;  Fig.  S3, 
a  perfect  animal  developed  on  a  ring. 


148  INDIVIDUALITY  IN  ORGANISMS 

unite  smoothly,  and  from  this  new  tissue  all  gradations 
from  single  tentacles,  through  groups  of  tentacles  of 
various  sorts  up  to  complete  small  anemones  (Figs. 
81-83)  arise. 

The  various  tentacle  groups  in  Figs.  81  and  82  and 
the  individual  in  Fig.  83  are  made  up  of  cells  which  are 
descended  from  both  apical  and  basal  ends  of  the  piece 
and  a  more  or  less  definite  new  individuation  occurs 
in  these  cells.  There  can  be  little  doubt  that  in  these 
cases  the  origin  of  these  various  degrees  of  individuation 
is  associated  with  the  growth  of  new  tissue  at  the  line  of 
union  between  the  cut  surfaces.  The  metabolic  rate  in 
this  tissue  is  higher  than  in  the  other  regions  of  the  piece, 
and  if  it  is  enough  higher  the  new  tissue  becomes  inde- 
pendent and  produces  a  new  apical  region,  or  some  part 
of  it,  according  to  conditions.  Wherever,  about  the 
circumference,  growth  of  new  tissue  is  most  rapid  and 
extensive,  there  the  new  individual  is  most  likely  to  arise. 
Often  it  is  possible  to  determine  beforehand  the  region 
of  the  circumference  where  such  tentacle  groups  or 
individuals  shall  arise,  by  making  the  outline  of  one  or 
both  cut  surfaces  irregular  at  some  point  or  making  a 
number  of  small  cuts  near  together  in  them.  In  such 
'regions  there  is  more  growth  of  new  tissue  and  a  new 
gradient  and  new  individual  are  more  likely  to  arise. 

As  regards  the  minor  axes,  it  is  of  great  interest  to 
note  the  wide  range  of  variations  which  occurs.  Many 
bilaterally  as  well  as  radially  symmetrical  and  asym- 
metrical forms  appear  among  the  tentacle-groups,  and 
it  is  evident  that  the  symmetry  of  the  groups  is  in  many 
cases  related  to  the  line  of  union  and  not  to  any  pre- 
existing symmetry  of  the  parent  animal.     In  these  rings 


THE  RANGE  OF  DOMINANCE  149 

we  see  new  individuals  being  localized  and  develo[)in^' 
where  it  is  impossible  to  conceive  of  any  internal  local- 
izing and  determining  factors  other  than  quantitative 
metabolic  conditions. 

In  the  case  of  Planaria  I  have  been  able  to  increase 
the  frequency  of  biaxial  heads  (see  Fig.  48,  p.  99)  in  very 
short  pieces  by  partially  narcotizing  the  animals  before 
cutting  and  keeping  the  pieces  in  a  dilute  solution  of  a 
narcotic,  e.g.,  chloretone,  for  a  day  or  two  before  allow- 
ing them  to  develop.     Under  such  conditions  the  meta- 
boHc  rate  in  the  pieces  is  of  course  decreased,  and  so 
dominance  in  the  direction  of  the  original  gradient  ii 
still  further  decreased.     Consequently,  when  the  pieces 
are  returned  to  water  and  allowed  to  develop,  the  con- 
ditions are  even  more  favorable  for  the  establishment  of 
the   reversed   gradient  at   the   basal   end,   and  biaxial 
structures  develop  in  a  larger  percentage  of  cases  than 
when  the  pieces  are  not  narcotized.     The  effect  of  the 
narcotic  is  simply  to  aid  in  decreasing  the  dominance  of 
the  original  apical  region  of  the  piece  and  so  to  increase 
the   probability   of   the   establishment   of   an   effective 
reversed   gradient   and   dominance   at   the   basal   end. 
This  experiment  has  not  as  yet  been  attempted  with 
Tubularia,  but  will  no  doubt  be  successful  with  proper 
technique. 

THE  EXTENSION  OF  DOMINANCE  DURING  DEVELOPMENT 

That  the  range  of  dominance  undergoes  extension 
during  development  is  evident  from  many  facts.  In  the 
young  Planaria,  for  example,  a  second  zooid  arises  at 
the  posterior  end  of  the  body  when  the  animal  is  less  than 
five  millimeters  in  length,  i.e.,  the  range  of  dominance 


I50  INDIVIDUALITY  IN  ORGANISMS 

at  this  stage  of  development  is  only  three  or  four  milli- 
meters.^ In  the  adult  animal,  however,  the  range  of 
dominance  as  indicated  by  the  length  of  the  first  zooid 
may  be  ten  or  twelve  millimeters  or  even  more  under 
certain  conditions.  Evidently  with  advancing  differen- 
tiation of  the  nervous  system  the  conductivity  has 
increased,  and  so  the  transmission-decrement  has  be- 
come less  and  the  range  of  transmission  greater. 

In  Stenostomum  also  the  more  advanced  the  devel- 
opment of  a  zooid,  the  greater  the  distance  from  its 
head-region  at  which  the  head-region  of  a  new  zooid 
is  determined,  as  will  appear  by  reference  to  Fig.  29 
(p.  81).  Other  animal  forms  which  undergo  agamic 
reproduction  show  similar  relations,  and  it  is  also 
probable  that  the  increasing  capacity  for  co-operation 
and  control  of  parts  with  advancing  development,  so 
far  as  it  depends  on  the  nervous  system,  results  to 
some  extent  from  the  increase  in  efficiency  of  trans- 
mission, though  various  other  factors  may  also  be 
concerned. 

In  plants  also  similar  relations  appear.  In  the  dif- 
ferentiated part  of  the  plant  stem  the  range  of  domi- 
nance of  a  bud  or  a  growing  tip  over  others  is  very  nmch 
greater  than  in  the  embryonic  region  of  the  growing  tip, 
but  their  later  development  is  inhibited  by  the  growing 
tip  as  a  whole,  even  though  further  growth  has  greatly 
increased  the  distance  between  them.  The  dominance 
of  the  growing  tip  as  a  whole  has  a  much  greater  range 
in  the  differentiated  parts  of  the  plant  than  the  domi- 
nance of  its  apical  region  over  much  nearer  parts  in  the 

» Child,  "Studies  on  the  Dynamics  of  Moiphogenesis.  Ill," 
Jour,  of  Ex  per.  ZooL,  XI,  igii. 


THE  RANGE  OF  DOMINANCE 


^5' 


embryonic  or  slightly  differentiated  tissue  of  the  grow- 
ing tip  itself. 

In  the  higher  animals  the  extension  of  dominance  is 
evidently  very  much  greater  than  in  the  lower  forms. 
In  the  medullated  nerve  fibers  of  the  higher  vcrtcl^rates 
the  transmission-decrement  is  so  slight  that  some  authors 
have  denied  its  existence.  Various  lines  of  experiment 
have  indicated,  however,  that  a  transmission-decrement 
does  exist  even  in  vertebrate  nerves  (see  pp.  173-75). 
Tashiro  has  shown  that  a  gradient  in  carbon-dioxide 
production  exists  in  nerve  fibers,  and  I  have  observed  a 
distinct  susceptibility  gradient  in  certain  nerves.  The 
nerve  is  essentially  a  specialized  protoplasm  which 
conducts  with  less  decrement  and  therefore  to  greater 
distances  than  other  kinds  of  protoplasm,  and  the 
central  nervous  system  arises  in  those  regions  of  the 
body  where  the  transmitted  changes  primarily  originate. 

The  extension  of  dominance  during  the  development 
of  the  higher  animals  is  so  great  that  the  range  of  domi 
nance  is  undoubtedly  very  much  greater  than  the  size 
of  the  individual.  In  these  forms  individual  size  is 
limited,  not  by  the  range  of  dominance,  but  by  the 
decrease  in  metabolic  rate  which  accompanies  the  pro- 
gressive differentiation,  and  so  limits  growth.  Onl\- 
in  early  stages  of  development,  or  in  the  lower  organ- 
isms, where  nerves  are  either  absent  or  not  very  good 
conductors,  does  the  size  of  the  individual  ecjual  tlu' 
range  of  dominance. 

EXPERIMENTAL   PHYSIOLOGICAL   ISOLATION   AND 
REPRODUCTION   IN   PLANTS 

The  course  of  development  in  the  single  plan! 
individual  suggests  the  dominance  of  the  growing  tip 


152 


INDIVIDUALITY  IN  ORGANISMS 


of  the  stem,  but  physiological  isolation  of  parts  and 
reproduction  of  new  individuals  afford  the  only  means 
of  demonstrating  experimentally  the  existence  of  domi- 
nance and  its  varying  range.  From  among  the  accu- 
mulated data  concerning  what  the  botanists  commonly 
call  correlation,  a  few  simple,  well-known  experiments 

are    briefly    described    to 


A 


Figs.  84,  85. — Diagrammatic 
outlines  of  leguminous  seedlings, 
illustrating  effect  of  removal  of 
growing  tip:  Fig.  84,  uninjured 
seedling;  Fig.  85,  development  of 
shoots  from  axils  of  cotyledons 
after  removal  of  stem-tip. 


show  how  readily  physio- 
logical isolation  and  repro- 
duction may  be  brought 
about  in  plants. 

The  young  seedling  of 
a  leguminous  plant  (pea, 
bean)  possesses  the  general 
form  indicated  diagram - 
matically  in  Fig.  84.  The 
further  normal  develop- 
ment of  the  stem  consists 
primarily  in  its  elongation 
and  the  development  of 
leaves  by  the  activity  of 
the  growing  tip  at  its 
apical  end,  but  if  this 
growing  tip  is  removed  a 
new  growing  tip,  or  in  some 


cases  more  than  one,  arises 
from  the  axillary  region  of  each  cotyledon,  as  indicated 
in  Fig.  85.  These  axillary  shoots  very  rarely  appear 
when  the  original  growing  tip  is  present  and  active,  but 
their  development  results  regularly  from  its  removal. 
If  both  of  the  shoots  grow  at  about  the  same  rate  they 
may  both  continue  to  develop  and  so  give  rise  to  two 


THE  RANGE  OF  DOMINANCE  153 

stems,  each  of  the  same  character  as  the  single  stem  in 
normal  plants,  but  if  one  grows  more  rapidly  the  growth 
of  the  other  is  usually  soon  inhibited  and  only  the  one 
continues  to  develop.  If,  instead  of  removing  the 
primary  growing  tip,  we  inhibit  its  metabolic  activity 
in  any  way  without  killing  it  or  injuring  it  otherwise,  the 
result  is  the  same  as  if  it  were  removed.  Inclosure  of 
the  primary  growing  tip  in  plaster  of  paris  or  in  an 
atmosphere  of  hydrogen  accomplishes  this  result  without 
injury,  for  it  is  capable  of  resuming  growth  after  removal 
of  the  plaster  or  return  to  air.  If  the  primary  tip  is 
inhibited  in  this  way  until  the  axillary  shoots  have 
appeared  and  is  then  allowed  to  resume  its  activity,  the 
growth  of  the  axillary  shoots  is  in  turn  inhibited  and 
the  primary  stem  continues  its  development,  unless  the 
axillary  shoots  have  attained  a  length  two  or  three  times 
as  great  as  that  of  the  main  stem  before  the  inhibition  of 
the  primary  tip  is  removed.  In  that  case  the  further 
growth  of  the  primary  tip  may  be  almost  entirely 
inhibited  by  the  axillary  shoots,  and  it  may  even  die, 
while  they,  or  one  of  them,  as  the  case  may  be,  continue 
development.  Many  modifications  of  the  experiment 
are  possible  at  different  stages  of  development  and  in 
different  plants.  In  stems  with  lateral  buds,  such  as  the 
willow,  if  the  apical  growing  tip  is  removed  the  uj^jx-r- 
most  lateral  bud  or  buds  will  develop  and  their  develoj) 
ment  inhibits  the  development  of  those  lower  down, 
if  we  remove  them  or  prevent  their  development  b>' 
inclosing  them  in  plaster,  the  buds  next  below  will 
develop,  and  so  on. 

In   many   plants   removal   or   inhibition   of   all    tlii 
growing  stem- tips  present  results  in   the   tormation  «)! 


154  INDIVIDUALITY  IN  ORGANISMS 

so-called  '' adventitious"  buds,  which  may  arise  from 
differentiated  cells,  as  in  the  case  of  the  begonia  (Figs. 
38,  39),  and  may  be  scattered  irregularly  over  various 
parts  of  the  plant  according  to  the  conditions  of  the 
experiment.  Often  the  presence  of  a  single  one  of  the 
original  buds  is  sufficient  to  inhibit  the  formation  of 
these  adventitious  buds.  The  appearance  of  adventi- 
tious buds  on  plants  in  nature  is  usually  due  to  the 
weakening  of  existing  growing  tips  through  advancing 
age  or  injury  of  some  sort. 

Such  adventitious  buds  very  often  arise  in  large 
numbers  simultaneously  without  any  regular  arrange- 
ment with  reference  to  each  other.  The  absence  of 
definite  space  relations  in  such  cases  is  undoubtedly 
due  to  the  fact  that  they  arise  simultaneously,  or  nearly 
so.  Various  cells  here  and  there  which  happen  to  have 
a  slightly  higher  metabolic  rate  than  others  begin  to 
develop  into  new  buds  at  about  the  same  time;  conse- 
quently none  of  the  buds  is  dominant  over  the  others. 
If,  however,  one  of  the  adventitious  buds  gets  a  start 
beyond  the  others  in  any  way,  it  inhibits  the  further 
development  and  may  even  bring  about  the  death  of 
others  within  a  certain  distance  of  it.  Moreover,  where 
a  gradient  is  present  in  the  part  on  which  the  buds  ap- 
pear, so  that  one  or  more  buds  appear  first  in  a  certain 
region — the  region  of  highest  metabolic  rate  in  the  part — 
they  inhibit  the  growth  of  others  within  a  certain  dis- 
tance or  throughout  the  part. 

In  various  conifers  the  dominance  of  the  growing  tip 
of  the  main  stem  appears  in  a  somewhat  different  form. 
In  these  trees,  as  long  as  the  growing  tip  of  the  main 
stem  is  present  and  active,  lateral  branches  arise  radially 


THE  RANGE  OF  DOMINANCE  155 

around  the  main  stem  and  grow  outward  from  the  trunk, 
and  the  branches  of  the  second  order  arise  in  most  cases 
more  or  less  bilaterally  on  them.  Removal  of  the  main 
growing  tip  is  followed  by  the  bending  upward  of  one  or 
more  of  the  uppermost  lateral  branches,  further  growth 
in  the  vertical  direction,  and  radial  instead  of  bilateral 
outgrowth  of  new  branches.  Here  one  or  more  of  the 
lateral  branches  nearest  the  upper  end  of  the  stem 
react  to  the  absence  of  the  main  growmg  tip  by  changing 
direction  and  form  of  growth  to  that  characteristic  of  tin- 
original  tip.  If  this  branch  is  removed,  branches  farther 
down  the  trunk  react  in  the  same  way. 

According  to  most  authorities,  dominance  of  one 
part  over  another  is  effective  only  or  chiefly  in  one 
direction  along  the  stem,  namely,  from  the  apical  end 
downward.  Buds  or  growing  tips  at  or  nearer  the 
apical  end  are  capable  of  inhibiting  buds  farther  down 
the  stem,  but  the  latter  are  not  capable  or  are  less 
capable  of  inhibiting  the  former.  In  recent  experi- 
mentation,^ however,  it  has  been  demonstrated  that 
these  relations  may  be  reversed,  and  that  if  shoots  lower 
down  are  allowed  to  grow  for  a  long  enough  time  and  to  a 
large  enough  size,  while  buds  higher  up  are  inhibited 
by  artificial  means,  the  lower  shoots  sooner  or  later 
acquire  the  abihty  to  inhibit  the  higher  ones  after  the 
removal  of  the  artificial  inhibition.  This  is  what 
might  be  expected  if  inhibition  depends  on  the  relations 
of  metaboHc  gradients.  Under  ordinary  conditions 
the  upper  levels  of  the  stem  represent  higher  levels  in 
the  gradient  and  therefore  inhibit  or  obliterate  gradients 

'W.  Mogk,  "  Untersuchungen  iiber  Korrelationen  von  Knospcn 
und  Sprossen,"  Archiv  fur  Entwickelungsjnechanik,  XXXMII,  1914. 


156  INDIVIDUALITY  IN  ORGANISMS 

lower  down  more  reiidily  than  these  with  their  lower 
rate  are  able  to  reverse  the  whole  established  proto- 
plasmic gradient  higher  up.  If,  however,  a  new  gradient 
at  a  lower  level  becomes  established  while  the  dominant 
region  above  is  inhibited,  it  is  conceivable  that  it  may  in 
time,  by  its  gradual  extension  in  the  stem,  obliterate 
more  or  less  completely,  or  perhaps  reverse,  the  original 
gradient  and  so  dominate  regions  higher  up,  at  least  to 
some  extent.  This  is  apparently  the  case  in  the  seedling 
mentioned  above  (p.  153)  when  the  axillary  shoots  are 
allowed  to  grow  long  enough  while  the  main  growing 
shoot  is  inhibited.  Under  such  conditions  they  are 
apparently  able  to  inhibit  what  was  originally  the  domi- 
nant region  of  the  whole  plant. 

It  is  often  possible  to  isolate  a  part  of  the  plant  from 
the  dominance  of  the  gro^ving  tip  merely  by  cutting  the 
vascular  bundles  connecting  the  two  parts.  The  devel- 
opment of  buds  on  the  leaves  of  certain  plants  may  be 
induced  by  severing  the  chief  vein  or  veins  of  the  leaf, 
other  tissues  remaining  intact.  In  such  cases  buds 
appear  peripheral  to  the  cut,  usually  near  the  veins, 
but  in  some  plants  on  the  leaf  margins. 

The  inhibiting  influence  is  not  confined  to  the  grow- 
ing  tips  of  stems,  for  it  has  been  shown  that  a  leaf  plays 
a  part  in  inhibiting  the  growth  of  the  bud  in  its  axial. 
Removal  of  the  leaf  or  inhibition  of  its  activity  may 
bring  about  outgrowth  of  the  bud,  if  the  inhibition 
from  other  sources  is  not  too  complete.  In  certain  cases 
it  has  been  shown  that  one  part  of  a  leaf  may  inhibit 
other  parts.  In  Cyclamen  persicum,  for  example,  the 
young  seedling  (Fig.  86)  possesses  at  first  only  a  single 
leaf,   one   of    the    cotyledons.     Removal    or    inhibition 


THE  RANGE  OF  DOMINANCE 


157 


by  inclosure  in  plaster  of  the  distal  part  of  the  blade  of 
this  leaf  before  its  growth  is  completed  is  followed  by  the 
development  of  a  new  leaf  surface  from  each  side  of  the 
basal  portion,  as  in  Fig.  87.  Wlien  the  whole  bhidc  of 
the  leaf  is  cut  off  or  inhibited,  the  margms  of  the  petiole 
just  below  the  level  of  the  cut  give  rise  to  a  separate  new 
leaf  on  each  side  (Fig.  88).  Here  the  basal  portion  of 
the  leaf  and   the  distal  region  of   the  petiole  margin 


Figs.  86-88. — Dominance  and  physiological  isolation  in  leaf  of 
Cyclamen  persicimi:  Fig.  86,  intact  seedling  (from  Hildebrand);  Fig.  87, 
development  of  new  leaf  blade  from  each  side  of  leaf  base  after  removal 
of  more  apical  portion;  Fig.  88,  development  of  new  leaf  from  each  side 
of  petiole  margin  after  removal  of  whole  leaf  (from  Goebel). 

evidently  possess  the  capacity  to  develop  as  a  leaf,  but 
are  prevented  from  doing  so  as  long  as  the  original  leaf 
or  its  distal  portion  is  present  or  active. 

Attention  has  been  called  to  the  fact  that  roots, 
wherever  they  appear  on  the  plant,  are  apparently 
subordinate,  specialized  individuals  and  originate  in 
definite  relations  to  parts  which  represent  regions  or 
levels  physiologically  less  remote  than  the  root-tij) 
from  a  stem- tip  or  bud  (see  pp.  104,  105).     Alost  plants 


158 


INDIVIDUALITY  IN  ORGANISMS 


with  roots  possess,  however,  not  a  single  root,  but  a 
root  system  which  is  a  composite  individual,  each  root 
representing  a  single  constituent  individual.  In  such  a 
root  system  relations  of  dominance  and  subordination 
similar  to  those  in  stem  systems  exist.  The  formation  of 
each  new  root  represents  a  reproduction  and  the  estab- 
lislnnent  of  a  new  root  individual.  In  plants  possessing 
a  single  main  root  with  lateral  roots  arising  from  it  (Fig. 
84)  this  relation  appears  very  clearly.  As  the  main  root 
grows  in  length  directly  downward,  lateral  roots  arise 


Figs.  89-91. — Effects  of  removal  or  inhibition  of  main  root-tip  on 
direction  of  growth  of  lateral  roots  (from  Bruck). 

successively  at  a  certain  distance  from  its  growing  tip 
and  grow  obliquely  downward  or  almost  horizontally. 
Experiments  with  seedhngs  show  that  if  the  growing  tip 
of  the  main  root  is  cut  off,  new  lateral  roots  arise  in 
larger  numbers  or  nearer  the  end  of  the  main  root,  and 
one  or  more  of  these  nearest  the  cut  end  grows  more 
nearly  in  the  vertical  direction  downward  than  when  the 
main  growing  tip  is  present  (Figs.  89,  90),  the  behavior 
differing  somewhat  according  to  the  level  of  the  cut. 
Apparently  in  these  seedlings  the  lateral  roots  which 


THE  RANGE  OF  DOMINANCE  159 

have  already  developed  do  not  change  their  direction 
of  growth  when  the  chief  growing  tip  is  cut  off;  only 
those  which  develop  after  the  operation  react,  but  they 
or  some  of  them  develop  as  main  instead  of  lateral  roots 
and  later  themselves  give  rise  to  lateral  roots.  If  the 
outgrowth  of  new  roots  near  the  cut  surface  is  inhibited 
after  the  removal  of  the  main  growing  tip  by  inclosing 
this  region  of  the  main  root  in  plaster,  roots  which  arise 
above  the  inhibited  region  may  react  by  growing  more 
directly  downward,  provided  they  are  not  too  far  away 
from  the  cut  surface  (Fig.  91).  The  lateral  roots  which 
react  in  this  way  to  the  absence  of  the  main  growing 
tip  resemble  more  or  less  closely  the  main  root  in  their 
later  development.  When  the  growing  tips  of  all  roots 
are  cut  off,  adventitious  roots  arise,  usually  in  large 
numbers  and  without  any  definite  order,  on  the  parts 
remaining.  Evidently  the  relation  between  the  con- 
stituent parts  of  the  root  system  is  a  relation  of  domi- 
nance and  subordination  like  that  in  the  stem  system. 

The  root  system  as  a  whole  seems  to  exert  an  inhibit- 
ing influence  on  the  development  of  roots  in  other  parts 
of  the  plant.  When  the  whole  root  system  is  removed 
or  its  metabolic  activity  inhibited,  new  roots  usually 
develop  from  the  basal  region  of  the  stem  if  external 
conditions  permit  their  growth  there;  if  not,  they  may 
appear  higher  up  on  the  stem.  The  propagation  of 
plants  by  cuttings  depends  on  this  ability  to  produce 
roots  on  the  stem  in  the  absence  of  the  root  system.  In 
an  experiment  described  by  Goebel  and  represented 
diagrammatically  in  Fig.  92,  a  bean  seedling  was  placed 
in  nutritive  solution,  b,  which  was  kept  at  low  tempera- 
ture, whereby  the  activity  of  the  root  system  was  largely 


i6o 


INDIVIDUALITY  IN  ORGANISMS 


inhibited.  A  part  of  the  stem  was  then  surrounded  with 
water,  a,  at  ordinary  temperature  to  provide  the  mois- 
ture necessary  for  the  growth  of  roots,  and  roots  arose 

on  this  region.  Submer- 
ging part  of  the  stem  in 
water  in  this  way  does  not 
result  in  the  development 
of  roots  when  the  original 
root  system  is  active.  By 
inclosing  a  region  of  the 
stem  in  a  chamber  con- 
taining ether  vapor,  and 
thus  anesthetizing  but  not 
killing  it,  McCallum  was 
able  to  induce  the  forma- 
tion of  roots  above  the 
anesthetized  region,  as  in- 
dicated in  Fig.  93.  In  this 
experiment  the  original 
root  system  was  present 
and  uninjured,  but  the  re- 
gion above  the  anesthe- 
tized level  was  apparently 
cut  off  from  its  influence, 
and,  the  moisture  being 
sufficient,  new  roots  ap- 
peared near  the  basal  end. 
These  experiments  with  roots  seem  to  indicate  that 
not  only  does  a  relation  of  dominance  and  subordination 
exist  between  the  different  parts  of  a  root  system,  but 
that  the  root  system  as  a  whole  dominates  the  stem  to  a 
certain  extent,  so  far  as  the  production  of  roots  is  con- 


FiGS.  92,  93. — Diagrammatic 
figures  illustrating  experiments  on 
root  production  on  the  stems  of 
seedlings;  only  lower  parts  of 
plants  shown:  Fig.  92,  formation 
of  roots  on  stem  at  a  when  this 
region  is  kept  moist  after  inhibi- 
tion of  original  root  system,  b,  by 
low  temperature  (after  Goebel); 
Fig.  93,  formation  of  roots  above 
a  region  of  stem  inclosed  in 
narcotic  atmosphere  (after 
McCallum's  description). 


THE  RANGE  OF  DOMINANCE  i6i 

cerned.  If  this  dominance  and  the  dominance  of  the 
stem-tip  both  result  from  metaboHc  gradients,  then 
there  must  be  in  plants  possessing  roots  two  metabolic 
gradients  in  opposite  directions,  the  apical  region  of  onv 
being  in  the  stem-tip  or  tips,  that  of  the  other  in  tlu- 
root-tip  or  tips. 

Two  gradients  in  opposite  directions  along  the  same 
axis  cannot  exist  at  the  same  time  without  interfering 
with  and  partially  obliterating  each  other  unless  they 
have  different  paths  of  transmission  or  are  of  different 
metabolic  character.  Concerning  the  possibility  of  the 
simultaneous  transmission  of  different  metabolic  changes 
in  different  directions  in  the  same  protoplasm  we  know 
nothing,  and  our  knowledge  of  conducting  paths  in  the 
plant  does  not  go  far  beyond  the  fact  that  some  part  ot 
the  vascular  bundles  seems  to  transmit  some  kind  of 
change  better  than  other  tissues. 

It  is  possible,  however,  that  the  influence  of  the  root 
system  on  the  stem  as  a  whole  may  be  different  in 
character  from  the  dominance  of  the  main  root-tip  on 
lateral  roots.  This  possibility  is  suggested  by  the  fact 
that  the  range  of  dominance  within  the  root  system  is 
rather  short,  even  where  the  tissues  are  differentiated, 
while  the  apparent  dominance  of  the  root  system  as  a 
whole  over  the  stem  and  other  parts  of  the  plant  is 
apparently  unlimited  in  range  or  without  relation  to 
distance.  The  root  system  takes  up  water  and  nutri- 
tive salts  and  these  are  transported  to  other  i)arts  of 
the  plant.  It  is  conceivable  that  the  inhibiting  inOu- 
ence  of  the  root  system  on  the  formation  of  roots  in  other 
parts  of  the  plant  may  be  rather  a  transportative  than  a 
transmissive  correlation,  and  that  the  other  parts  give 


1 62  INDIVIDUALITY  IN  ORGANISMS 

rise  to  roots  when  this  transportation  falls  below  a 
certain  minimum  or  when  they  are  isolated  from  it  in  any 
way.  This  alternative  is  more  nearly  in  accord  with  the 
views  of  most  botanists,  and  it  seems  at  present  more 
satisfactory  than  the  assumption  of  two  opposed  and 
overlapping  gradients.  If,  however,  this  relation 
between  root  system  and  other  parts  is  transportative 
rather  than  transmissive,  McCallum's  experiment  de- 
scribed above  of  bringing  about  physiological  isolation  of 
the  upper  levels  of  the  stem  from  the  root  system  by 
local  anesthesia  seems  to  indicate  that  the  transportation 
is  not  a  simple  physical  process  but  is  dependent  in 
some  way  and  to  some  extent  upon  the  metabolic 
activity  of  living  cells. 

If  we  accept  this  alternative  and  admit  at  the  same 
time  the  primary  dominance  of  the  stem- tip  or  tips  and 
the  secondary  dominance  within  the  root  system  of  the 
root-tip  or  tips  we  must  regard  the  root  system  as  a  sub- 
ordinate specialized  constitutent  individual  of  the  com- 
posite plant  individual.  The  root,  like  the  leaf,  is 
primarily  determined  by  relations  to  other  parts  of  the 
plant,  but  requires  certain  external  conditions  for  its 
development  and  differentiation.  Like  the  leaf  also,  the 
root  or  root  system  shows  a  certain  degree  of  second- 
ary individuation  among  its  parts. 

The  formation  of  roots  is  the  reaction  of  a  plant 
individual  to  a  certain  relation  between  internal  and 
external  conditions,  and  this  relation  may  apparently 
be  brought  about  either  by  the  inhibition  of  activity  in, 
or  absence  of,  the  original  root  system,  or  in  many  cases 
by  changes  in  the  external  conditions,  such  as  decrease 
in   light   and   increase   in   moisture,   even   though   the 


THE  RANGE  OF  DOMINANCE  163 

original  root  system  is  present.  The  root  of  the  plant, 
like  the  basal  end  of  the  animal  body,  is  the  morpho- 
logical expression  of  the  performance  of  a  certain  func- 
tional activity  primarily  subordinate  to  and  dependent 
upon  the  activities  of  other  parts.  Without  the  activi- 
ties of  parts  representing  higher  levels  in  the  primary 
gradient,  root  formation  does  not  occur,  but  when  it  has 
occurred  the  products  of  the  special  metabolic  activity 
of  roots  transported  to  other  parts  affect  the  metabolic 
processes  there  and  so  inhibit  more  or  less  effectively  the 
formation  of  roots  there. 

From  this  point  of  view  the  apparent  dominance  of 
the  root  system  over  other  parts  of  the  plant  with  respect 
to  root  formation  is  not  a  feature  of  the  primary  and 
fundamental  relation  of  dominance  and  subordination 
in  the  individual,  but  rather  a  secondary  relation — trans- 
portative  rather  than  transmissive — unlike  the  primary 
relation,  and  resulting  from  local  differentiation  which 
is  itself  associated  with  and  dependent  upon  the  primary 
relation. 

THE   LOCALIZATION   OF  EXPERIMENTAL  REPRODUCTION  IN 
RELATION   TO   DIFFERENT   AXES 

It  is  often  possible  to  alter  the  localization  of  the 
new  dominant  region  in  the  reconstitution  of  an  isolated 
piece  by  altering  the  gradient  relations  of  the  piece.  A 
few  examples  from  the  flatworm,  Planar ia,  among 
the  animals  and  the  liverwort,  Marchantia,  among  the 
plants  will  illustrate  the  point. 

It  has  been  pointed  out  (pp.  80,  81)  that  the  out- 
growth of  new  tissue  on  a  piece  of  Planaria  isolated  by 
transverse  planes  of  section  is  most  rapid  in  the  median 


164 


INDIVIDUALITY  IN  ORGANISMS 


ventral  region  of  the  apical  end,  this  region  represent- 
ing the  region  of  highest  metabolic  rate  or  irritability 
resultant  from  the  three  main  axial  gradients.  By  alter- 
ing the  shape  of  the  piece  in  relation  to  the  axial  gradients 
it  is  possible  to  alter  the  position  of  this  outgrowth  and 
so  the  position  of  the  new  head.  In  a  piece  cut  very 
obliquely  {abed,  Fig.  94),  the  head  develops  as  in  Fig.  95, 
and  the  side  of  the  head  which  arises  from  the  more 


98 


Figs.  94-98. — Localization  of  head-formation  in  the  reconstitution 
of  pieces  of  Planaria  as  resultant  of  apico-basal  and  transverse  axial 
gradients:  Fig.  94,  diagrammatic  outline  of  part  of  body  of  Planaria, 
indicating  shapes  of  pieces;  Fig.  95,  asymmetrical  position  of  head  in 
reconstitution  of  piece,  abed;  Fig.  96,  reconstitution  of  piece,  aehd; 
Fig.  97,  reconstitution  of  piece,  aegi;  Fig.  98,  reconstitution  of  piece,  afi. 

apical  level  of  the  piece  is  likely  to  develop  somewhat 
more  rapidly  than  the  other  side.  This  asymmetry 
of  position  and  development  is  due  largely  to  the  fact 
that  one  side  of  the  cut  surface  represents  a  higher  level 
in  the  major  axial  gradient  than  the  other  and  so  reacts 
more  rapidly.  When  the  cut  surface  is  oblique,  the 
major  gradient  becomes  a  factor  in  determining  the 
position  of  most  rapid  dedifferentiation,  division,  and 
new    development    of    cells,    and    this    determines    the 


THE  RANGE  OF  DOMINANCE  165 

position  of  the  new  head.  In  a  piece  achd.  Fig.  94, 
the  head  develops,  as  shown  in  Fig.  96,  on  the  apical  cut 
surface,  but  in  a  shorter  piece  aegi,  Fig.  94,  the  head  is 
likely  to  appear  at  an  angle  to  the  apical  and  median  cut 
surfaces,  as  in  Fig.  97.  This  condition  results  when  the 
metaboHc  rate  of  the  cells  on  the  median  cut  surface  is  as 
high  as  that  of  the  cells  on  the  apical  cut  surface,  so  that 
both  take  an  equal  part  in  giving  rise  to  the  new  head. 
In  pieces  like  afi,  Fig.  94,  the  head  often  develops  nearly 
or  quite  in  the  direction  of  the  transverse  axis  (Fig.  98). 
In  such  pieces  there  is  little  difference  in  metabolic  rate 
between  apical  and  basal  cut  surfaces,  and  the  cuts  are 
not  sufficiently  oblique  so  that  the  higher  level  in  the 
major  gradient  of  the  lateral  as  compared  with  the 
median  region  of  the  cut  surface  overbalances  its  lower 
level  in  the  transverse  gradient.  Consequently  the 
median  regions  of  both  cut  surfaces  represent  the  region 
of  highest  rate  or  irritability  in  such  a  piece  and  therefore 
become  the  head-forming  region.  For  these  and  many 
other  experimental  modifications  of  the  position  of  the 
head  in  reconstitution  no  satisfactory  general  basis  of 
interpretation  has  heretofore  been  discovered,  but  I 
know  of  no  case  which  cannot  be  very  simply  accounted 
for  in  terms  of  axial  metabolic  gradients. 

In  the  bilaterally  symmetrical  liverwort  MarcJiautui 
(Fig.  23,  p.  78),  the  gradient- relations  are  apparently  very 
similar  to  those  in  Planaria.  In  these  plants  practically 
every  cell  of  the  body  is  capable  of  giving  rise  to  a  new 
plant,  but  in  pieces  without  the  growing  tip  new  growing 
tips  originate  in  definite  relations  to  the  axes,  and  their 
presence  inhibits  the  formation  of  others.  In  general, 
on  transverse  cut  surfaces  new  individuals  arise,  like 


i66  INDIVIDUALITY  IN  ORGANISMS 

the  head  in  Planaria,  in  or  near  the  median  ventral 
region  of  the  apical  end  of  the  piece  just  basal  to  the 
cut  surface  (Fig.  99).  When  the  piece  is  taken  from  the 
lateral  margin  of  the  plant  body  and  does  not  contain 
the  median  region,  individuals  usually  arise  near  the 
apical  end  and  ventrally  on  the  most  nearly  median 
region  of  the  piece  (Fig.  100).  In  pieces  with  oblique 
instead  of  transverse  apical  cut  surfaces  the  position 
of  the  new  individual  varies  according  as  the  piece 
contains  part  of  the  midrib  or  not,  according  to  the 
obliquity  of  the  plane  of  the  cut,  and  probably  also 
according  to  the  region  of  the  body.  Where  the  piece 
does  not  include  the  midrib  the  new  individual  usually 
arises  ventrally  near  the  inost  apical  region  of  the  piece, 
the  major  gradient  being  the  chief  factor  in  determining 
its  position.  Thus  in  Fig.  loi  the  new  plant  appears 
near  the  lateral  margin,  undoubtedly  because  the  meta- 
bolic level  is  higher  here  than  elsewhere.  The  con- 
ditions here  are  apparently  much  like  those  which 
determine  the  asymmetrical  position  of  the  new  head  in 
Planaria  in  Fig.  95.  In  pieces  which  contain  a  part  of 
the  midrib  this  is  usually  the  chief  factor  in  determining 
the  position  of  the  new  head.  The  piece  in  Fig.  102, 
for  example,  is  cut  from  one  side  of  the  body  and  includes 
part  of  the  midrib  at  the  basal  end  of  the  oblique  cut,  and 
the  new  bud  arises  here.  The  influence  of  the  midrib 
in  localization  in  this  form  depends  on  the  fact  that  the 
cells  in  this  region  retain  their  capacity  for  growth  and 
division  much  longer  than  the  cells  of  the  lateral  regions, 
and  so  they  represent  a  relatively  high  metabolic  level 
and  bear  much  the  same  relation  to  the  transverse 
gradient  that  the  apical  growing  tip  does  to  the  major 


THE  RANGE  OF  DOiMINANCE 


167 


gradient.  Because  of  the  relatively  high  metabolic  1l'\-l1 
of  these  cells  along  the  midrib  this  region  [)la>'s  a  more 
important  part  in  the  localization  of  reproduction  than 
the  median  region  in  Planaria.  In  fact,  the  experi- 
mental evidence  seems  to  indicate  that  the  chief  differ- 
ence in  axial  relations  between  Marchantia  and  Planaria 
is  the  higher  metabolic  level  of  the  apical  region  of  the 
transverse  gradient,  the  median  region  of  the  body. 


100 


99 


Figs.  99-102. — Localization  of  new  individual  as  resultant  of  dilTer- 
ent  axial  gradients  in  pieces  of  liverwort,  Marchantia:  Fig.  99,  usual 
position  in  median  ventral  region  near  apical  end  of  piece;  Figs,  loo- 
102,  different  positions  of  new  individual  apparently  determined  by  the 
different  relations  of  the  axial  gradients  according  to  shape  of  piece 
and  region  represented  (from  Vochting). 

With  advancing  age  the  region  of  the  midril)  undergoes 
gradual  differentiation  and  so  loses  to  a  greater  or  less 
extent  its  high  metabolic  rate. 

These  experiments  and  many  others  which  cannot  be 
discussed  here  are  highly  significant  in  that  they  indi- 
cate the  essential  identity  in  character  of  the  dilTcrent 
axes  of  the  physiological  individual.  In  fact,  I  believe 
they  constitute  evidence    of    the    greatest   importance 


i68  INDIVIDUALITY  IN  ORGANISMS 

for  the  fundamentally  quantitative  character  of  at  least 
the  main  axes  of  the  body,  for  if  the  different  axes  are 
qualitatively  different,  I  cannot  conceive  how  the 
position  of  a  new  head  or  growing  tip  on  an  isolated 
piece  can  be  determined  in  one  case  chiefly  by  the  major 
axis,  in  another  as  a  resultant  of  two  or  more  axes,  and 
in  a  third  by  one  of  the  minor  axes.  If,  however,  all 
axes  are  fundamentally  gradients  in  metabolic  rate,  the 
facts  are  very  simply  accounted  for,  as  1  have  tried  to 
show.  The  major  axis  is  the  major  axis,  not  because 
its  nature  is  fundamentally  different  from  that  of  other 
axes,  but  because  it  arises  first  or  because  its  apical  region 
has  the  highest  metabolic  rate  of  any  part  of  the  body, 
and  the  minor  axes  are  minor  axes  because  they  arise 
later  or  their  apical  regions  have  a  lower  rate.  When 
the  major  gradient  is  in  any  way  obliterated  to  a  suffi- 
cient degree  one  of  the  minor  gradients  may  act  in 
exactly  the  same  way  as,  though  often  more  slowly 
than,  the  major  gradient  where  it  is  present.  This 
is  true,  of  course,  only  for  forms  and  stages  in 
which  the  fundamental  quantitative  character  of  the 
axes  has  not  been  too  greatly  altered  by  progressive 
differentiation. 

The  fact  that  a  plant  bud  may  be  inhibited  by  the 
main  growing  tip,  by  another  bud,  by  a  growing  leaf,  or 
by  a  lateral  branch  also  indicates  that  there  is  nothing 
specifically  different  in  these  different  inhibitions  and  so 
suggests  that  these  different  plant  axes  act  in  essentially 
a  quantitative  way  in  dominating  other  parts.  One 
may  be  substituted  for  the  other  without  altering  the 
character  of  the  effect. 


THE  RANGE  OF  DOMINANCE  169 

CONCLUSION 

It  is  possible  to  control  ;and  alter  experimentally 
the  spatial  relations  of  parts  in  the  individual  by  altering 
the  length  of  the  metabolic  gradient  and  so  the  range 
of  dominance.  Parts  of  the  individual  may  come  to  lie 
beyond  the  range  of  dominance  in  consequence  of 
increase  in  size  of  the  whole,  of  decrease  in  range  and 
degree  of  dominance  by  decrease  in  the  metabolic  rate 
in  the  dominant  region,  of  decrease  in  conducti\ity  of 
the  paths  of  correlation,  and  of  the  direct  local  action 
of  external  factors  which  increase  the  independence  of 
subordinate  parts.  Parts  thus  physiologically  isolated 
may  reproduce  new  individuals  if  the  essential  axial 
gradients  exist,  or  arise  in  them.  In  many  of  the  lower 
organisms  the  original  axis  or  axes  may  be  experi- 
mentally obliterated  and  a  new  axis  and  dominance 
established  in  relation  to  external  conditions  which 
determine  differences  in  metabolic  rate  in  different  parts 
of  the  mass.  In  general,  the  range  of  dominance 
increases  during  the  development  of  the  individual 
because  the  conductivity  of  the  protoplasm  increases, 
and  special  conducting  paths  develop  as  the  m()ri:)ho- 
logical  expression  of  the  fundamental  correlative  con- 
ditions in  the  individual.  The  essentially  quantitative 
character  of  different  axes  of  the  individual  is  indicated 
by  the  fact  that  one  axis  may  be  experimentally  substi- 
tuted for  another  in  determining  the  localization  of  a  new 
individuation. 


CHAPTER  VI 

DISCUSSION,  CONCLUSIONS,  AND  SUGGESTIONS 
THE   NATURE    OF   DOMINANCE 

It  has  been  assumed  thus  far  that  dominance  depends 
on  a  transmitted  change,  or  excitation,  rather  than  on 
the  transportation  of  substance,  and  it  now  becomes 
necessary  to  consider  what  basis  there  is  for  this  con- 
clusion. As  already  pointed  out  (pp.  26,  27),  some 
sort  of  organization  must  be  present  in  order  that  trans- 
portative  or  chemical  correlation  may  occur  in  a  definite 
and  constant  manner.  If  different  regions  of  the  body 
produce  specifically  different  substances  they  must  be 
specifically  different,  and  if  these  substances  act  on 
certain  other  parts  in  a  definite  specific  way  those  parts 
must  possess  a  certain  constitution.  The  data  of 
experimental  reproduction  discussed  in  earlier  chapters 
show  that  new  individuals  arise  from  parts  of  old  indi- 
viduals which  either  cannot  possibly  possess  the  "organi- 
zation" of  a  complete  individual  or  must  possess  an 
indefinite  number  of  such  organizations.  The  latter 
alternative  leads  to  a  conception  of  the  Weismannian 
sort,  and  I  have  tried  to  indicate  how  unsatisfactory 
such  conceptions  are  (pp.  22,  23). 

If,  on  the  other  hand,  the  individual  is  primarily 
a  metabolic  gradient  in  a  specific  protoplasm,  the  only 
primary  difference  between  the  dominant  and  other 
levels  of  the  gradient  is  a  difference  of  metabolic  rate. 
At  this  time  the  products  of  metabolism  at  different 

170 


CONCLUSIONS  AND  SUGGESTIONS  171 

levels  of  the  gradient  are  not  specifically  different,  but 
differ  in  quantity.  If  the  transportation  of  chemical 
substances  is  the  only  means  of  correlation  between 
the  different  levels  of  the  gradient,  it  is  impossible  to 
understand  either  how  the  gradient  can  persist  or  how 
a  relation  of  dominance  and  subordination  can  arise 
between  ievels  of  higher  and  those  of  lower  metabolic 
rate.  Specific  chemical  correlation  between  parts  is 
possible  only  when  specifically  different  parts  are  present, 
and  the  definite  space  relations  which  we  find  associated 
with  physiological  dominance  do  not  usually  appear  in 
such  correlation.  In  short,  I  beheve  it  is  impossible 
to  conceive  of  the  process  of  organic  individuation  with 
the  definite,  constant,  and  orderly  character  which  it 
actually  possesses  as  having  its  origin  in  transportativc 
or  chemical  correlation  alone. 

If,  however,  the  metabolic  gradient  arises  and  is 
maintained  by  the  transmission  of  excitation  from  the 
region  of  highest  metabolic  rate,  this  region  becomes 
dominant  simply  because  its  metabolic  rate  is  so  high 
that  it  determines  and  maintains  the  gradient  in  rate, 
and  the  differences  in  rate  at  different  levels  bring  about 
sooner  or  later  differences  in  constitution  and  character 
of  the  protoplasmic  substratum.  In  regions  of  high 
rate  only  certain  relatively  stable  substances  remain  as 
constituents  of  the  substratum,  and  others  are  broken 
down  and  eliminated.  In  regions  of  lower  rate,  on  the 
other  hand,  other  substances  accumulate  as  parts  of 
the  substratum  because  under  these  conditions  tliey  are 
less  readily  or  less  rapidly  broken  down  than  where  the 
rate  is  higher,  and  it  is  also  probable  that  the  character 
of  synthesis  differs  with  the  rate  of  metabolism.     In 


172  INDIVIDUALITY  IN  ORGANISMS 

this  way  each  level  of  the  gradient  develops  a  character- 
istic protoplasm  and  the  character  of  the  protoplasm 
in  turn  modifies  and  alters  the  character  of  the  reactions, 
and  so  specific,  or  what  we  call  qualitative,  differences 
arise,  and  different  specific  substances  may  be  produced 
at  different  levels  of  the  gradient.  At  the  moment  when 
these  specific  differences  first  appear  chemical  cbrrelation 
in  the  commonly  accepted  sense  becomes  possible,  and 
from  this  time  on  it  may  play  a  part  in  determining  the 
character  of  further  changes  at  the  various  levels.  After 
chemical  correlation  appears  it  is  unquestionably  a 
factor  of  great  importance  in  determining  the  character 
of  the  various  parts  and  so  of  the  individual  as  a  whole. 
The  point  which  I  wish  to  emphasize  is  that  chemical 
or  transportative  correlation  does  not  and  cannot 
account  for  the  origin  of  the  individual,  because  the 
individual  must  exist  as  some  sort  of  orderly  and  definite 
relation  or  organization  before  orderly  and  definite 
chemical  correlation  between  its  parts  is  possible.  The 
dynamic  conception  of  the  individual  is  primarily  con- 
cerned, not  with  the  orderly  specificities  of  chemical 
correlation,  but  with  the  conditions  in  protoplasm  which 
make  those  orderly  specificities  possible. 

The  occurrence  of  transmission  in  living  protoplasm 
is  a  familiar  fact.  The  existence  of  a  transmission- 
decrement  and  therefore  of  a  limited  range  of  effect- 
iveness has  been  demonstrated  for  the  transmission 
of  stimuli  in  plant  tissues  and  in  various  animal  nerves. 
In  many  of  the  lower  animals  the  range  of  effectiveness 
in  transmission  can  readily  be  observed  by  means  of 
the  range  of  reaction  to  stimuli  of  different  intensity. 
In  transportative  correlation  a  definite  range  of  effective- 


CONCLUSIONS  AND  SUGGESTIONS  173 

ness  cannot  exist  unless  transportation  is  uniform  and 
constant  in  rate  in  all  parts  at  each  level  and  the  sub- 
stance is  gradually  destroyed  or  transformed  during' 
the  transportation.  The  dynamic  theory  affords  an 
adequate  basis  for  the  very  definite  range  of  dominance 
which  we  find  in  organisms,  and  a  chemical  theory 
does  not. 

Tashiro's  recent  investigations  on  carbon-dioxide 
production  and  my  observations  on  susceptiljility 
gradients  in  the  nerve  indicate  that  physiological  domi- 
nance in  the  neuron,  i.e.,  the  direction  of  transmission,  is 
associated  with  the  existence  of  a  metabolic  gradient. 
Individuation  in  what  is  probably  the  most  highl\' 
specialized  cell  individual  in  the  organism  apparenll\- 
starts  from  the  same  condition,  the  metabolic  gradient, 
as  in  the  simplest  axiate  animal  or  plant.  It  is  certain 
that  dominance  in  the  neuron  depends  primarily  on 
transmission  and  not  on  transportation.  This  argu- 
ment from  the  highly  specialized  to  the  simi)le  is 
perhaps  not  of  great  value;  still  I  camiot  but  beheve 
that  the  existence  of  an  axial  gradient  in  metabolic 
rate  in  the  neuron  and  in  the  simple  axiate  indi\'iduals 
among  the  lower  organisms  is  a  fact  of  real  significance. 

It  has  been  very  generally  beHeved  by  ph}'si(>logists 
that  the  nerve,  at  least  the  meduUated  nerve  of  verte- 
brates, transmits  excitations  under  normal  conditions 
without  a  decrement  in  energy  or  intensity.  It  is. 
however,  a  well-known  fact  that  even  in  these  nerves  a 
decrement  appears  when  transmission  takes  place  at 
low  temperature  or  in  partially  narcotized  or  com- 
pressed nerves;  in  fact,  under  various  conditions  whicii 
decrease   metabolic   rate   or   irritability   in    the   nerve. 


174  INDIVIDUALITY  IN  ORGANISMS 

Those  who  hold  that  the  nerve  in  normal  condition 
transmits  without  a  decrement  have  usually  maintained 
that  under  depressing  conditions  the  nerve  behaves 
in  a  different  way  from  the  normal  nerve  and  that  the 
decrement  exists  only  under  these  conditions.  In  view 
of  the  fact  that  in  the  nerves  of  the  lower  animals  a 
transmission-decrement  undoubtedly  occurs  normally, 
and  that  in  protoplasmic  transmission  in  the  absence  of 
nerves  the  decrement  is  even  more  marked,  the  grounds 
for  the  belief  that  transmission  without  a  decrement 
occurs  in  the  vertebrate  nerve  do  not  appear  to  be  ade- 
quate. It  seems  scarcely  probable  that  the  higher 
degree  of  specialization  of  the  vertebrate  nerve  has 
brought  about  a  fundamental  change  in  the  character 
of  transmission  of  such  a  nature  that  the  decrement  is 
reduced  to  zero  and  transmission  to  an  indefinite  or 
infinite  distance  is  possible.  The  experiments  along 
this  line  prove  only  that  with  the  very  limited  lengths  of 
nerve  available  the  decrement  under  normal  conditions 
is  very  slight  or  inappreciable.  Evidently  the  nerve 
of  the  vertebrate,  and  particularly  of  the  higher  verte- 
brate, is  a  much  better  conductor  than  undifferentiated 
protoplasm  or  even  than  the  nerves  of  lower  animals,  and 
within  the  limits  of  the  individual  vertebrate  body  the 
decrement  is  undoubtedly  slight  or  practically  absent 
when  the  nerve  is  in  good  metabolic  condition,  but  the 
conclusion  that  there  is  no  decrement  in  such  cases 
seems  unwarranted.  It  is  also  highly  improbable  that 
the  nature  of  transmission  in  the  cooled,  partially 
narcotized,  or  compressed  nerve  is  essentially  different 
from  that  in  the  same  nerve  under  normal  conditions, 
and  since  a  decrement  appears  under  depressing  condi- 


CONCLUSIONS  AND  SUGGESTIONS  175 

tions,  the  only  conclusion  justified  by  the  facts  seems 
to  be  that  a  decrement  must  exist  in  normal  transmissicjn, 
but  is  much  less  marked,  and  the  range  of  transmission 
is  therefore  much  greater,  than  under  depressing  con- 
ditions. Undoubtedly  in  the  higher  animals  the  range 
of  transmission  is  very  much  greater  than  the  limits  of 
the  individual  body,  for  the  size  of  the  individual  in 
these  forms  is  limited  by  other  factors  than  the  range 
of  dominance  (see  pp.  46,  47,  151),  but  that  transmission 
mthout  decrement  occurs  is  far  from  being  demon- 
strated and,  as  I  have  endeavored  to  show,  there  is 
much  evidence  against  such  a  view. 

It  is  also  a  highly  significant  fact  that  the  nervous 
system,  which  is  the  chief  conducting  organ  of  the  body 
in  those  forms  which  possess  it,  develops  in  a  definite 
relation  to  the  axial  gradients.  The  dominant  region 
of  the  nervous  system  appears  in  the  apical  region  of 
the  major  axial  gradient,  and  at  other  levels  of  the  body 
which  contain  the  central  nervous  system  it  represents 
the  region  of  highest  metabolic  rate  in  the  minor  gradi- 
ents. If  the  unity  of  the  organism  depends  primarily 
upon  transportation,  there  is  no  apparent  reason  why 
it  should  change  to  a  unity  depending  on  transmission 
or  why  the  dominant  region  of  the  central  nervous 
system  should  arise  in  the  dominant  region  of  the 
primitive  individual.  If,  however,  organic  unity  is  funda- 
mentally and  from  the  beginning  dependent  upon  trans- 
mission, the  general  plan  and  arrangement  of  the  nervous 
system  are  very  evidently  the  expression  in  speciaHzcd 
structure  and  function  of  the  primary  unity  and  relation 
which  was  the  starting-point  of  individuation,  and  domi- 
nance  or   control    by  nervous    transmission   is   mercl)' 


176  INDIVIDUALITY  IN  ORGANISMS 

a  specialized  and  more  effective  modification  of  the 
dominance  which  is  the  foundation  of  organic  unity  and 
order. 

Moreover,  the  nervous  system  dominates  or  controls 
the  chemical  activities  of  the  organism  to  a  very  con- 
siderable degree.  If  the  primary  dominance  is  purely 
a  matter  of  chemical  correlation,  it  is  difficult  to  con- 
ceive how  the  functional  dominance  of  the  nervous 
system  has  come  about,  but  if  the  primary  dominance 
depends  upon  transmission  of  the  same  general  char- 
acter as  nervous  transmission,  the  functional  dominance 
of  the  nervous  system  is  the  natural  and  necessary 
consequence  of  the  primary  relations. 

As  regards  the  role  of  the  nervous  system  in  develop- 
ment and  reconstitution,  there  has  been  much  differ- 
ence of  opinion.  Many  biologists  have  maintained 
that  the  nervous  system  exerts  a  specific  formative 
influence  on  various  parts  and  so  determines  their 
course  of  development  and  differentiation,  while  others 
deny  the  existence  of  any  such  influence.  In  the  case 
of  certain  organs  and  parts,  e.g.,  striated  muscle,  it  has 
been  definitely  demonstrated  that  embryonic  develop- 
ment may  occur  without  nervous  connection,  but  in 
the  mature  condition  frequent  nervous  stimulation  is 
necessary  for  maintenance  of  structure  and  function. 
And  as  regards  reconstitution,  some  investigators  have 
found  that  certain  parts,  such  as  the  amphibian  leg, 
regenerate  incompletely  or  not  at  all  in  the  absence  of 
nerves,  while  others  have  maintained  that  connection 
with  nerves  is  unnecessary  for  complete  regeneration 
of  these  parts.  These  apparently  contradictory  and 
confusing  results  can,  I  believe,  be  very  simply  inter- 


CONCLUSIONS  AND  SUGGESTIONS  177 

preted  and  harmonized.  If  the  metabolic  rate  in  the 
organ  or  part  in  question  is  sufficiently  hi^^h,  it  is  ca- 
pable of  undergoing  its  characteristic  development  and 
differentiation  without  nervous  stimulation,  assuming 
of  course  that  its  other  relations  as  a  part  of  the  indi- 
vidual are  not  fundamentally  altered;  but  when  its 
intrinsic  metabolic  rate  falls  below  a  certain  level  its 
development  does  not  occur,  or  is  incomplete,  or  it 
undergoes  atrophy  unless  its  rate  is  further  increased 
by  nervous  stimulation.  In  the  case  of  striated  muscle 
during  the  earlier  stages  of  development  the  intrinsic 
metabolic  rate  is  high  enough  to  permit  without  nervous 
stimulation  the  accumulation  of  structural  material 
and  the  characteristic  course  of  differentiation  deter- 
mined by  other  correlative  conditions,  but  as  differ- 
entiation and  senescence  progress  the  metabolic  rate 
falls,  and  finally  the  muscle  is  not  even  able  to  maintain 
itself  in  the  absence  of  the  accelerating  influence  of 
nervous  stimulation  upon  its  metabolic  rate,  because 
when  its  rate  falls  below  a  certain  level  it  does  not  replace 
its  losses  by  new  muscle  substance.  In  the  regenera- 
tion of  the  amphibian  leg  and  other  cases  where  the 
influence  of  the  nervous  system  is  in  dispute,  the  relations 
are  without  doubt  essentially  the  same. 

There  is  no  reason  to  believe  that  the  nerve  impulse 
is  anything  more  than  an  acceleration  of  metabolism. 
The  appearance  of  the  nervous  system  does  not  consti- 
tute the  addition  of  something  new  to  the  organism;  il 
is  merely  the  visible  expression  of  relations  already 
existing  and,  as  the  facts  indicate,  of  the  relations 
which  constitute  the  foundation  and  starting-point  of 
individuation. 


178  INDIVIDUALITY  IN  ORGANISMS 

The  question  whether  metabolic  gradients  involving 
different  metabolic  processes  may  exist  at  the  same  time 
in  the  same  protoplasm  must  at  least  be  raised.  So  far 
as  gradients  depending  on  transmission  are  concerned, 
this  question  is  really  the  question  whether  different 
sorts  of  changes  or  excitations  may  be  transmitted 
through  the  same  protoplasm  and  whether  different 
metabolic  effects  result.  Any  answer  to  this  question 
at  present  is  little  more  than  a  guess.  It  is  perhaps 
conceivable  that  at  least  in  undifferentiated  or  slightly 
differentiated  protoplasm  some  degree  of  difference  in 
the  character  of  the  transmitted  change  may  exist  under 
different  conditions  of  excitation,  etc.  If  such  differ- 
ences do  exist,  they  must  of  course  be  important  factors 
in  development  and  differentiation,  but  they  merely 
complicate  and  do  not  alter  fundamentally  the  character 
of  unity  and  order  in  the  individual.  At  present  there 
seems  to  be  no  real  evidence  that  they  exist. 

THE   NATURE    OF   INHIBITION 

In  chaps,  iv  and  v,  I  have  pointed  out  that  the 
inhibition  or  retardation  of  new  individuation  by  the 
dominant  region  of  an  individual  occurs  when  the  origi- 
nal gradient  is  sufficiently  fixed  in  the  protoplasm,  or  the 
metabolic  rate  at  the  levels  concerned  is  sufficiently  high 
to  prevent  the  establishment  of  a  gradient  in  another 
direction  or  to  obliterate  more  or  less  completely  or 
prevent  the  further  development  of  a  gradient  in  another 
direction.  In  Tubularia  the  inhibiting  influence  of  the 
apical  region  on  the  development  of  a  hydranth  at  the 
basal  end  of  a  piece  is  apparently  simply  the  obhterating 
effect  of  the  original  gradient  on  the  gradient  in  the 


CONCLUSIONS  AND  SUGGESTIONS  179 

opposite  direction.  If  the  latter  attains  a  sufTicicntly 
high  rate  it  interferes  with  or  obhterates  the  other  and 
the  hydranth  develops,  though  partial  inhibition  may 
be  evident  in  its  shortness  and  slow  development. 

In  the  case  of  a  lateral  bud  of  a  plant,  the  develop- 
ment of  which  is  inhibited  by  the  main  growing  tip, 
the  relation  is  probably  the  same.     As  long  as  the  bud 
is  within  the  range  of  dominance  of  the  growing  tip  its 
own  gradient  from  apex  to  base  is  more  or  less  com- 
pletely obliterated  by  a  gradient  from  base   to  apex 
determined  by   the  main    growing    tip.     This  may  in 
time  alter  the  protoplasmic  gradient  in  the  bud  deter- 
mined in  the  earlier  stages  of  its  individuation  so  that 
it  becomes  incapable  of  development  or  develops  only 
into  a  short  branch,  a  spine,  or  some  other  rudimentary 
structure.     It  is  interesting  to  note  that  Mogk  in  his 
studies  of  plant  correlation  finds  that  when  the  axillary 
shoots  of  a  seedling  are  allowed   to  grow  until    they 
attain  dominance  over  the  main  shoot  (see  pp.  152,  153), 
the  latter  often  dies  and  the  death  gradient  is  in  the 
reverse  direction  from  that  of  death  from  lack  of  water 
or  other  conditions  in  an  uninhibited  shoot.     Leaves 
and  roots  probably  represent  partially  inhibited  gradi- 
ents under  certain  conditions,  and  some  of  the  specialized 
outgrowths  on  the  animal  body,  such  as  appendages,  may 
perhaps  in  some  cases  represent  somewhat  similar  rela- 
tions, though  I  know  of  no  definite  evidence  bearing  on 
this  point. 

So  far  as  the  evidence  goes,  it  indicates  that  all 
inhibition  of  this  sort  is  a  matter  of  interference  between 
gradients  in  opposite  or  nearly  opposite  directions,  the 
one  gradient  reducing,  obhterating,  or  even  reversing 


i8o  INDIVIDUALITY  IN  ORGANISMS 

the  other.  This  interference  is  in  certain  respects 
analogous  to  physical  interference  in  the  transmission 
of  water  waves,  sound  waves,  light  waves,  etc.,  but  the 
protoplasmic  substratum  in  the  organism  represents  a 
factor  not  concerned  in  physical  interference  in  non- 
solid  media.  Undoubtedly  a  gradient  which  is  originally 
dynamic  becomes  more  or  less  stably  fixed  or  estab- 
lished in  the  protoplasm  as  a  gradient  in  irritability, 
structure,  or  differentiation,  because  the  effects  of  the 
transmitted  excitations  modify  the  protoplasmic  condi- 
tion and  this  modification  may  become  more  or  less 
persistent.  Temporary  inhibition  may  result  from 
temporary  interference  between  metabolic  gradients, 
but  for  permanent  or  long-enduring  inhibition  the 
protoplasmic  condition  determined  by  one  gradient 
must  be  reduced  or  obliterated  or  its  direction  reversed 
by  the  action  on  the  protoplasm  of  another  gradient. 
In  the  cases  of  obliteration  or  reversal  of  the  axial 
gradients  by  other  gradients  this  factor  undoubtedly 
plays  a  more  or  less  important  part,  and  the  increasing 
stability  of  the  protoplasmic  substratum  with  the  prog- 
ress of  individual  development  and  evolution^  deter- 
mines that  such  obliteration  and  reversal  occur  much 
more  readily  in  the  lower  than  in  the  higher  organisms. 
Since  conduction  in  the  nerve  is  apparently  asso- 
ciated with  an  axial  gradient,  it  is  at  least  an  interesting 
question  whether  nervous  inhibition  may  not  be  funda- 
mentally a  similar  relation  of  gradients,  either  in  differ- 
ent neurons  or  in  the  innervated  organ.  The  mechanism 
of  nervous  inhibition  is  still  obscure,  but  if  the  nervous 

'  Child,  Senescence  and  Rejuvenescence,  1015,  pp.  50,  53,  194,  267, 
463--6S- 


CONCLUSIONS  AND  SUGGESTIONS  i8i 

system  is  really  the  final  expression  of  the  primitive 
dominance  in  the  individual,  it  is  conceivable  that  the 
highly  specialized  nervous  inhibition  may  have  some- 
thing in  common  with  the  primitive  form  of  inhibition 
in  the  lower  animals  and  plants. 

ORIGIN    OF    METABOLIC    GRADIENTS    AND    OF    DOMINANCE 

The  data  of  reconstitution  discussed  in  chaps,  iv 
and  v  show  very  clearly  that  new  metabolic  gradients 
arise  in  relation  to  various  external  factors:  in  Tuhularia 
the  cut  end  (pp.  132-37);  in  Corymorpha  the  difference 
between  a  free  surface  and  one  in  contact  (pp.  142-46); 
in  Harenactis  difference  in  the  character  of  a  wound 
determining  more  or  less  growth  of  new  tissue  and  so 
the  localization  of  a  new  apical  region.  As  regards  the 
plants,  the  evidence  from  adventitious  buds  (pp.  83-86) 
also  indicates  that  the  axes  of  such  buds  arise  anew, 
slight  differences  in  metabolic  rate  between  different 
cells  apparently  often  determining  whether  a  new  indi- 
vidual shall  arise  in  one  place  or  another.  As  regards 
various  plants,  we  know  that  certain  of  the  minor  axes, 
and  in  some  cases  the  major  axis,  are  determined  by 
the  differential  action  of  light.  I  believe  we  are  justi- 
fied in  saying  that  whenever  a  new  metabolic  gradient 
of  sufhciently  high  rate  is  established  by  an  external 
factor  a  new  individuation  occurs. 

It  is  of  course  easy  to  assume,  as  is  often  done,  that 
polarity  and  symmetry  are  self-determined  in  the  in- 
di\ddual,  and  that  these  self-determined  relations  are 
simply  altered  and  modified  by  external  factors.  But 
the  evidence  for  self-determination  is  lacking,  and  the- 
evidence   for   external   determination   is  abundant   and 


i82  INDIVIDUALITY  IN  ORGANISMS 

highly  conclusive.  The  assumption  of  self-determined 
polarity  and  symmetry  in  protoplasm  is  simply  super- 
fluous, and  the  burden  of  proof  is  upon  its  supporters. 

Of  course  the  metaboUc  gradients  present  in  one 
individual  may  persist  in  the  parts  when  that  individual 
divides,  so  that  in  such  cases  the  axial  relations  of  the 
new  individual  are  predetermined.  This  is  the  case  in 
fission  in  Planaria  (pp.  92-96)  and  in  many  other  forms. 
Apparently  also  the  gradient  in  a  reproductive  body, 
e.g.,  many  eggs,  is  often  determined  by  its  relations  of 
attachment,  nutrition,  etc.,  to  the  parent  body. 

In  pieces  of  Tubularia,  Corymorpha,  Planaria,  and 
many  other  forms,  the  original  polarity  gradually  dis- 
appears as  the  length  of  the  isolated  piece  decreases 
until  it  becomes  practically  apolar,  and  new  polarities 
arise  in  relation  to  conditions  at  the  cut  ends  (pp.  97-101). 
This  fact  indicates  that  polarity  is  rather  a  matter  of 
relation  of  parts  than  a  fundamental  property  of  pro- 
toplasm, for  in  fractions  of  the  axis  below  a  certain 
length  it  disappears. 

In  nature  a  particular  kind  of  individual  show^s  certain 
characteristic  axial  relations;  it  is  radially  or  bilaterally 
symmetrical,  or  a  combination  in  a  characteristic  way 
of  radial  and  bilateral  arrangements.  But  the  char- 
acteristic axial  relations  are  not  invariable;  they  appear 
regularly  merely  because  events  follow  the  same  course  in 
successive  generations.  In  plants  the  axial  relations 
can  be  altered  in  many  ways  and  by  many  external 
factors.  Bilateral  symmetry  may  be  transformed  into 
radial  or  radial  into  bilateral,  the  position  of  branches 
may  be  altered  from  alternate  to  opposite  or  to  whorled, 
and  so  on.     The  bilateral  tentacle  groups  on  the  rings 


CONCLUSIONS  AND  SUGGESTIONS  iS^ 

in  Harenactis  (Fig.  82,  p.  147)  show  that  the  radial 
arrangement  characteristic  of  the  animals  in  nature  is 
not  invariably  determined  in  the  protoplasm,  but  is 
only  one  of  various  possibilities,  which  may  or  may  ncjt 
be  realized  according  to  conditions. 

If  my  conception  of  the  relation  between  the  meta- 
bolic gradient  and  dominance  is  correct,  then  of  course 
the  origin  of  a  new  gradient  is  the  origin  of  a  new  domi- 
nance, and  if  such  a  gradient  is  uninhibited  by  gradients 
in  other  directions,  and  if  its  metabolic  rate  is  hi^^^h 
enough,  it  becomes  the  major  axis  of  an  individual  and 
its  region  of  highest  rate  the  dominant  apical  rcgUm. 

MORPHOLOGICAL   DIFFERENTIATION   IN  RELATION   TO 

METABOLIC   RATE 

The  belief  that  qualitative  differences  of  some  sort 
in  the  fundamental  constitution  of  the  organism  must 
underlie  the  morphological  and  physiological  differences 
which  arise  during  development  in  different  parts  of 
the  individual  has  been  so  widespread  among  biologists 
that  any  attempt  at  even  a  statement  of  the  problem 
of  differentiation  in  anything  like  quantitative  terms 
is  sure  to  meet  with  serious  objection  and  criticism  in 
some  quarters.  Nevertheless,  the  simplest  and  most 
satisfactory,  and,  I  believe,  the  only  adequate,  interpre- 
tation of  the  data  of  reconstitution  which  have  been 
discussed  in  preceding  chapters  is  that  the  starting- {)(jint 
of  differentiation  is  in  differences  in  metabolic  rate. 
The  attempt  to  interpret  these  facts  on  any  other  basis 
very  soon  becomes  involved,  either  in  the  barren  assump- 
tions of  the  hypotheses  which  simply  postulate  an 
invisible  organization  to  account  for  a  visible,  or  else 


i84  INDIVIDUALITY  IN  ORGANISMS 

in  the  equally  barren  neo-vitalistic  assumptions  of  some 
non-mechanistic  controlling  or  determining  principle, 
entelechy,  or  whatever  we  please  to  call  it. 

The  head  of  Planaria  will  serve  to  illustrate  the 
point.  I  have  shown  that  a  series  of  different  forms 
of  head  occur  in  reconstitution,  ranging  from  the  normal 
to  the  headless  condition  (pp.  106-8).  These  differ- 
ent forms  represent  various  degrees  of  inhibition  and 
they  result,  not  only  from  the  inhibitory  influence  of 
other  parts  (pp.  108-14),  but  can  be  produced  experi- 
mentally by  a  great  variety  of  conditions.  In  a  lot  of 
similar  pieces  from  animals  in  similar  physiological 
condition  a  decrease  in  head-frequency  or  a  shift  toward 
the  headless  condition  can  be  induced  by  low  tempera- 
ture, narcotics,  carbon  dioxide,  etc.,  although  in  certain 
cases,  as  we  have  seen  (pp.  11 2-13),  the  results  are  com- 
plicated by  the  metabolic  relations  between  the  head- 
forming  region  and  other  parts  of  the  piece.  On  the 
other  hand,  conditions  which  accelerate  metaboUsm, 
such  as  high  temperature  or  increased  motor  activity, 
increase  the  head-frequency  or  shift  it  toward  the  normal 
end  of  the  series.  We  cannot  believe  that  differences 
in  temperature  or  motor  activity  alter  the  fundamental 
"organization"  in  the  head-forming  region,  but  it  is 
a  fact  that  such  conditions  according  to  their  degree 
may  determine  any  or  all  of  the  various  kinds  of  head 
between  the  normal  and  headless  extremes. 

Again,  how  does  either  an  "organization"  or  an 
entelechy  aid  us  in  interpreting  the  structures  formed 
on  rings  in  Ilarenactis  (pp.  146-49)  ?  Here  results 
range  from  various  bilateral  arrangements  of  parts  to 
the    characteristic    radial    symmetry,    and    from    single 


CONCLUSIONS  AND  SUGGESTIONS  185 

tentacles  to  normal  animals.  Either  the  plan  of  or^^^ani- 
zation  or  the  purpose  of  entelechy  must  be  very  (lilTcrcnt 
in  different  tentacle  groups  on  such  rings.  \Vc  know, 
however,  that  the  pieces  will  not  form  rings  except 
under  certain  experimental  conditions,  and  that  when 
they  do  not  they  undergo  reconstitution  in  the  usual 
way  to  animals  of  the  usual  form.  Evidently  the 
development  of  these  structures  on  the  rings  results  from 
certain  experimental  conditions,  but  if  simple  experi- 
mental conditions  can  alter  the  fundamental  axial 
relations  in  the  individual,  what  is  the  necessity  of  the 
postulated  organization,  or  entelechy,  or  other  similar 
principle  ?  And  does  not  the  obhteration  in  Corymorpha 
of  the  original  axial  relations  and  the  establishment  of 
new  relations  in  their  place,  by  means  of  experimental 
conditions  whose  action  upon  metabolism  is  primarily 
quantitative  (pp.  142-46),  indicate  that  the  axes  them- 
selves are  primarily  quantitative  relations?  Similarly 
the  fact  that  the  localization  of  experimental  reproduc- 
tion may  be  determined  as  a  resultant  of  dilTerent  axes 
or  by  a  minor  axis  in  the  absence  of  the  major  axis 
(pp.  163-68)  forces  us  to  the  conclusion  that  the 
different  axes  are  fundamentally  identical  and  therefore 
represent  quantitative  relations. 

Moreover,  the  conception  of  the  organic  axis  as  a 
metabolic  gradient  enables  us  not  only  to  interpret,  but 
to  control  and  to  predict.  In  recent  work  on  the  oligo- 
chete  annelids,  by  Dr.  Hyman,  it  has  been  possible  on 
the  basis  of  the  metabolic  axial  gradient  to  predict  and 
control  experimental  results,  and  this  is  possible  among 
the  flatworms  to  an  even  greater  degree.  As  regards 
the  manner  in  which  physiological  and  morj^hological 


1 86  INDIVIDUALITY  IN  ORGANISMS 

specialization  results  from  difference  in  metabolic  rate 
there  are  various  possibilities.  In  a  physico-chemical 
complex  like  living  protoplasm  a  change  in  tempera- 
ture of  a  certain  amount  alters  the  rate  of  chemical 
reaction  to  a  certain  degree,  but  it  also  alters  many 
other  conditions  in  protoplasm,  e.g.,  osmotic  conditions, 
surface-tension,  aggregate  condition  of  colloids,  etc., 
and  it  alters  some  in  a  greater,  others  in  a  less,  degree. 
In  such  a  case  the  change  in  each  particular  process  or 
condition  in  the  living  protoplasm  may  be  quantitative, 
but  since  different  factors  are  altered  in  different  degree 
the  total  change  may  determine  qualitative  differences 
in  the  reactions  or  their  products.  Changes  of  this 
sort  may  result,  not  merely  from  differences  in  tempera- 
ture, but  from  other  primarily  quantitative  changes. 
In  fact,  it  is  very  doubtful  whether  we  can  alter  metabolic 
rate  to  any  great  extent  without  bringing  such  changes 
in  quality  somewhere  in  the  complex. 

Elsewhere  I  have  called  attention  to  various  facts 
which  have  as  yet  received  but  little  attention,  but 
which  indicate  that  a  relation  exists  between  morpho- 
logical structure  and  metabolic  rate.^  Structural  fea- 
tures which  are  stable  with  a  certain  metaboHc  rate  are 
eliminated  when  the  rate  increases,  while  decrease  in 
rate  may  determine  the  addition  of  new  structural  sub- 
stances, and  so  on.  Metabolic  rate  is  apparently  a 
factor,  though  of  course  by  no  means  the  only  one,  in 
determining  what  substance  or  substances  accumulate 
in  the  living  cell  as  structural  substratum,  and  the 
structural  substratum  is  an  important  factor  in  determin- 
ing the  character  of  the  reactions  which  occur  in  it. 

*  Child,  Senescence  and  Rejuvenescence,  iqis,  pp.  47-54.  226-27. 


CONCLUSIONS  AND  SUGGESTIONS  1S7 

The  lack  of  specificity  in  the  action  oi  a  great  variety 
ot  experimental  conditions  upon  development  and 
morphology  has  often  been  noted.  For  example,  the 
aberrations  or  abnormalities  in  development,  or  m(jre 
properly  the  partial  inhibitions  of  development  pro- 
duced by  low  temperature,  various  narcotics  and  poisons, 
and  many  other  conditions  are  essentially  the  same. 
The  reason  for  the  lack  of  specificity  undoubtedly  lies 
in  the  fact  that  the  action  of  these  various  substances 
and  conditions  is  primarily  quantitative,  yet  a  greater 
or  less  degree  of  cUfferentiation,  various  differences  in 
form  and  arrangement,  and  even  the  presence  or  absence 
of  specific  organs  may  be  determined  by  their  action. 

The  results  of  the  quantitative  changes  in  living 
protoplasm  in  a  particular  case  must  of  course  depend 
upon  its  specific  constitution.  The  kind  of  specializa- 
tion or  differentiation  which  arises  at  a  particular  level 
of  a  metabolic  gradient  must  depend  upon  this  constitu- 
tion, and  the  developmental  and  morphological  resem- 
blances between  different  forms  must  of  course  depend 
in  general  upon  similarities  of  constitution.  The 
development  of  the  region  of  highest  metabolic  rate 
in  the  major  gradient  as  a  growing  tip  in  plants  and  as 
a  central  nervous  system  or  brain  in  animals  must  result 
from  differences  in  constitution  and  dynamic  processes 
in  the  plant  and  animal  protoplasm,  but  growing  tii)s 
in  general  and  central  nervous  systems  in  general  have 
certain  common  characteristics. 

We  must,  I  beheve,  conclude  that  the  conception  of 
the  metabolic  gradient,  a  gradient  primarily  quantitative^ 
originating  in  and  primarily  determined  by  the  dominant 
region,  as  the  basis  of  physiological  and  morphological 


i88  INDIVIDUALITY  IN  ORGANISMS 

order,  of  "organization,"  specialization,  and  differentia- 
tion in  the  organic  individual,  not  only  presents  no 
fundamental  difficulties,  but  is  supported  by  a  great 
body  of  experimental  and  observational  evidence  from 
various  biological  fields. 

THE   FUNDAMENTAL  REACTION    SYSTEM 

If  the  dynamic  conception  of  the  organic  individual 
is  correct,  the  starting-point  hes,  not  in  a  certain  organi- 
zation, but  in  a  certain  reaction  system.  This  is  a 
protoplasm  of  specific  constitution  with  a  corresponding 
metabolic  specificity,  or  one  may  say  that  this  specificity 
is  the  expression  of  a  specific  constellation  of  conditions 
and  that  this  in  turn  has  been  determined  by  the  specific 
constellation  of  factors  external  to  itself  to  which  each 
organism,  individual,  or  part  has  been  subjected  in  the 
past.  It  is  this  reaction  system,  not  an  organization, 
which  constitutes  the  basis  of  inheritance,  and  it  is 
in  this  system  that  differences  in  metabolic  rate  initiate 
the  process  of  organization.  We  may  for  convenience 
regard  the  embryonic  or  undifferentiated  cell  of  the 
species  as  representing  this  fundamental  reaction  system, 
although  even  there  the  system  is  doubtless  not  reduced 
to  its  lowest  terms.  The  developmental  changes  in  this 
system  fall  into  two  groups,  the  self-determined'  changes 

^  It  is  perhaps  desirable  to  indicate  just  what  is  meant  by  self- 
determination  in  this  connection.  All  that  the  word  is  intended  to 
imply  here  is  that  the  region  of  highest  metabohc  rate  may  undergo 
certain  progressive  changes,  which  are  derermined  by  its  own 
constitution  and  by  continued  metabolism  in  it.  The*^e  changes  may 
in  time  make  this  region  different  structurally  and  physiologically 
from  what  it  was  originally,  even  though  it  is  independent  of  other 
parts. 


CONCLUSIONS  AND  SUGGESTIONS  189 

characteristic  of  the  dominant  region  and  the  correlatively 
determined  changes  characteristic  of  subordinate  regions. 
It  is  a  very  significant  fact  that  the  self-determined 
changes  in  animals  always  result,  where  they  proceed  far 
enough,  in  the  development  of  a  nervous  system.  Of 
course  as  a  matter  of  fact  the  changes  which  occur  in  the 
development  of  a  central  nervous  system  are  not  all  alj- 
solutely  self-determined,  for  if  they  were  all  cells  of 
the  nervous  system  would  be  alike.  We  may  say,  how- 
ever, that  in  the  animal  the  nervous  system  or  its  apical 
portion  represents  more  nearly  than  any  other  part  of 
the  body  the  result  of  self-determined  progressive  changes 
in  the  fundamental  reaction  system  of  the  species,  while 
other  parts  represent  the  result  of  changes  determined  by 
correlation  and  dependence^  From  this  point  of  view 
the  animal  organism  is  fundamentally  nervous  system; 
all  other  parts  represent  lower  levels  of  metaboHsm  and 
independence.  The  central  nervous  system  represents 
more  nearly  than  any  other  part  of  the  individual  the 
product  of  the  fundamental  reaction  system  at  its  highest 
level.  The  cephalic  nervous  system  is,  so  to  speak,  the 
organism  at  its  best. 

In  the  plant,  however,  the  self-determining  dominant 
region  remains,  at  least  during  growth,  in  an  unditTer- 
entiated  or  relatively  undifferentiated  condition  as  the 
growing  tip,  and  growth  and  cell  division  arc  its  chii'f 
activities.  In  consequence  of  this  condition  its  domi- 
nance over  other  regions  is  slight,  the  degree  of  indi- 
viduation in  the  plant  remains  low,  and  the  life  of  the 
plant  remains  simple  and  narrowly  limited  in  character. 

This  difference  between  animals  and  plants,  in  ilu- 
one  the  development  of  the  dominant  region  into  the 


190  INDIVIDUALITY  IN  ORGANISMS 

central  nervous  system,  the  most  stable  structure  physi- 
ologically of  the  body,  and  in  the  other  its  persistence 
indefinitely  as  an  embryonic  cell  or  a  group  of  cells,  must 
be  an  expression  of  the  fundamental  difference  between 
the  two  groups  of  organisms.  Evidently  this  difference 
is  primarily  a  difference  in  relation  between  the  proto- 
plasmic substratun^  and  the  metabolic  reactions.  Stable 
morphological  structure  and  differentiation  in  the  plant 
consist  largely  in  the  deposition  of  carbohydrates  and 
other  non-proteid  substances  within  or  about  the  cells, 
while  in  the  animal  morphological  differentiation  very 
generally  has  its  origin  and  foundation  in  the  accumula- 
tion and  specialization  of  protoplasm  itself.  Apparently 
the  protoplasmic  substratum  of  the  plant  is  much  less 
stable  physiologically  than  that  of  the  animal.  The 
plant  seems  to  be  incapable  or  almost  incapable  of  syn- 
thesizing proteid  molecules  which  are  physiologically 
stable  where  the  metabolic  rate  is  high.  The  protoplasm 
of  the  plant  cell  is  certainly  much  more  directly  and 
intimately  involved  in  the  chemical  reactions  of  metab- 
olism than  that  of  most  animal  cells;  consequently 
in  regions  of  high  metabolic  rate  no  persistent  proto- 
plasmic structure  like  that  of  the  animal  cell  can  arise, 
because  there  is  no  accumulation  of  relatively  stable 
substances  in  the  cell.  In  regions  where  the  metabolic 
rate  is  lower,  substances  may  accumulate  in  the  cell 
as  structure  which  with  a  higher  metabolic  rate  would 
be  decomposed.  In  the  plant,  therefore,  morphological 
differentiation  increases  with  increasing  distance  from 
the  growing  tip  and  decreasing  metabolic  rate,  while 
in  the  animal  differentiation  begins  and  is  most  stable 
in  the  apical  region — the  region  of  highest  reaction  rate — 


CONCLUSIONS  AND  SUGGESTIONS  191 

and  progresses  from  this  to  other  parts.  Animal  mctab- 
oHsm  evidently  synthesizes  highly  stable  molecules,  even 
where  metaboHc  acti\qty  is  most  intense. 

In  the  plant  the  whole  substratum  may  apparently 
be  mobilized  to  some  extent  when  the  metabolic  rate 
is  high,  and  only  as  the  rate  becomes  lower  do  substances 
accumulate  as  structure.  In  nearly  all  if  not  all  animals, 
on  the  other  hand,  certain  protoplasmic  substances  are 
relatively  more  stable  under  the  existing  metabolic 
conditions  than  in  the  plant  and  therefore  accumulate, 
and  a  progressive  structural  development  and  dilTer- 
entiation  occur  even  when  the  metabolic  rate  is  highest. 
In  the  animals  the  morphological  structure  which  de- 
velops in  the  region  of  highest  metabolic  rate  is  physio- 
logically the  most  stable  structure  of  the  body,  because 
the  less  stable  substances  are  decomposed  in  the  intense 
metabolic  activity  and  so  do  not  form  permanent  con- 
stituents of  the  substratum.  In  regions  of  lower  meta- 
bolic rate  substances  accumulate  which  are  readily 
removed  by  an  increase  in  metabolic  rate.  These  parts 
may  therefore  undergo  dedifferentiation  and  rcdiller- 
entiation.  The  head-region,  however,  or  more  specifi- 
cally, the  central  nervous  system,  is  almost  or  quite 
incapable  of  dedifferentiation  under  ordinary  conditions, 
because  its  structure  has  developed  under  conditions 
of  more  intense  metabolic  activity  than  any  other  part 
of  the  body  and  is  therefore  more  stable.  If  the 
metabolic  rate  could  be  increased  sufficiently  above  the 
rate  in  the  developing  nervous  system  without  bringing 
about  death,  doubtless  dedifferentiation  of  the  nervous 
system  would  occur  to  some  extent.  To  refer  brietly 
to  the  analogy  between  the  organism  and  the  flowing 


192  INDIVIDUALITY  IN  ORGANISMS 

stream  which  I  have  used  elsewhere/  the  plant  is  some- 
what like  a  stream  flowing  in  an  alluvial  channel,  capable 
of  shifting  and  removing  previous  structural  deposits, 
and,  when  its  rate  is  highest,  of  holding  all  its  sediment 
in  suspension.  The  animal,  on  the  other  hand,  repre- 
sents a  condition  like  that  in  the  stream  when  deposition 
of  sediment  is  going  on  and  giving  rise  to  stable  structure, 
even  where  the  rate  of  flow  is  highest.  In  such  a  stream 
the  most  stable  structure  develops  where  the  rate  of 
flow  is  highest,  while  the  structure  developed  with  a 
low  rate  of  flow  is  readily  altered  or  eliminated  by  an 
increase  in  rate. 

The  fundamental  differences  in  behavior  between 
plant  and  animal  are  of  course  associated  with  this 
difference.  Since  the  plant  is  to  a  large  extent  incapable 
of  developing  morphological  colloid  structures,  such  as 
nerve  and  muscle,  its  reactions  to  external  factors  are 
limited  very  largely  to  growth  reactions,  instead  of  being 
motor  reactions  like  those  in  most  animals.  The  low 
degree  of  individuation  and  physiological  efficiency  in 
the  plant  as  compared  with  the  animal  must  also  depend 
on  this  low  degree  of  physiological  stabihty  in  the  pro- 
toplasmic substratum. 

AGAMIC   REPRODUCTION   IN   RELATION   TO   PHYSIOLOGICAL 

ISOLATION 

The  occurrence  of  reproduction  in  consequence  of 
physiological  isolation  of  parts  under  experimental  con- 
ditions makes  it  highly  probable  that  at  least  many  of 
the  processes  of  agamic  reproduction  in  nature  are  like- 

^  Child,  "The  Regulatory  Processes  in  Organisms,"  Jour,  of 
Morphol.,XXll,  191 1. 


CONCLUSIONS  AND  SUGGESTIONS  IQ3 

wise  the  result  of  physiological  isolation.  Elsewhere  I 
have  endeavored  to  show  that  physiological  isolation  is 
a  fundamental  factor  in  asexual  reproduction  in  both 
plants  and  animals,  and  that  reproduction  results  from 
physiological  isolation  because  the  isolated  part  loses 
to  a  greater  or  less  extent  its  differentiation  as  a  part, 
becomes  physiologically  younger,  and  undergoes  a  new 
individuation.'  In  chap,  iv  above  it  was  also  pointed 
out  that  agamic  reproduction  in  Tubularia  and  Planaria 
is  readily  interpreted  as  the  result  of  physiological 
isolation.  Moreover,  in  the  discussion  of  the  data  of 
experimental  reproduction  we  have  seen  that  physio- 
logical isolation  and  reproduction  may  result,  not  only 
from  increase  in  size  beyond  the  range  of  dominance, 
but  also  from  decrease  in  the  range  of  dominance  in 
consequence  of  decrease  in  metabolic  rate  in  the  domi- 
nant region,  from  decrease  in  conductivity  in  the  path  of 
transmission,  and  finally  from  a  decrease  in  receptivity 
of  a  subordinate  part,  brought  about  by  the  action  of 
local  factors,  which  determine  the  establishment  of  new 
gradients  in  it  or  make  it  otherwise  more  independent. 
Undoubtedly  all  these  different  forms  of  physiological 
isolation  occur  in  nature,  and  in  many  reproductive 
processes  more  than  one  of  them  are  probably  concerned. 
Reproduction  in  consequence  of  increase  in  size  is 
one  of  the  commonest  forms  of  reproduction  in  organic 
individuals  from  the  single  cell  to  complex  organisms 
among  both  animals  and  plants.  Reproduction  also 
occurs  very  commonly  under  conditions  unfavorable  t») 

^  Child,  "Die  physiologische  Isolation  von  Teilen  des  OrKanismus." 
Vortrage  tend  Aujsatze  iiher  Entunckelungsmechanik,  H,  XI.  iqii;  Senes- 
cence and  Rejuvenescence,  1915,  PP-  228. 


194  INDIVIDUALITY  IN  ORGANISMS 

growth  or  active  life;  that  is,  under  conditions  which 
undoubtedly  decrease  metabolic  rate  and  so  decrease 
the  range  of  dominance.  Under  such  conditions  unicel- 
lular forms  often  fragment  into  a  number  of  small 
individuals,  and  some  of  the  simple  plants  break  up 
into  their  constituent  cells,  which  then  grow  and  divide 
to  form  small  individuals,  even  under  the  same  conditions 
which  made  impossible  the  persistence  of  the  original 
larger  individual.  Other  plants  give  rise  to  adventi- 
tious buds,  sometimes  in  great  numbers,  under  such 
conditions,  while  still  others  break  up  into  quiescent 
forms,  and  so  on.  In  my  study  of  senescence  and 
rejuvenescence  I  have  pointed  out  that  the  decrease  in 
metabolic  rate  with  advancing  senescence  in  the  lower 
animals  and  plants  often  leads  automatically  by  decreas- 
ing dominance  to  physiological  isolation  of  parts  and 
so  to  rejuvenescence  and  reproduction  of  new  individuals. 
'  Reproduction  under  depressing  conditions  has  often 
been  interpreted  in  a  teleological  way  as  an  attempt  of 
the  organism  to  avoid  extinction  by  producing  new 
individuals,  some  of  which  might  succeed  in  finding 
favorable  conditions  for  continued  existence.  As  a 
matter  of  fact,  however,  such  reproduction  is  merely 
the  expression  of  physiological  weakness;  the  individual 
can  no  longer  maintain  itself  as  a  unity  in  its  original 
size,  and  as  the  original  unity  disappears,  new  unities 
arise  as  local  metabolic  conditions  determine. 

Regarding  the  part  played  by  changes  in  the  con- 
ductivity of  the  path  of  transmission  in  bringing  about 
physiological  isolation  and  reproduction  in  nature,  we 
know  little.  It  is  undoubtedly  a  fact  that  the  increase 
in  conductivity  during  development  of  the  individual 


CONCLUSIONS  AND  SUGGESTIONS  195 

brings  about  an  extension  of  dominance  ami  so  inhibits 
or  retards  physiological  isolation  (see  pp.  149-51),  and 
it  is  probable  that  sooner  or  later  with  advancing 
senescence  a  decrease  in  conductivity  occurs  in  at  least 
some  cases.  It  is  also  probable  that  decrease  in  con- 
ductivity occurs  in  the  lower  organisms  under  external 
conditions  which  decrease  metabolic  rate  in  the  organism 
in  general.  Such  changes,  where  they  occur,  may  j)lay 
a  part  in  determining  physiological  isolation  and  repro- 
duction. 

Local  external  conditions  undoubtedly  assist  in  the 
physiological  isolation  of  subordinate  parts  in  man\' 
cases.  In  various  plants  local  conditions  very  favorable 
to  metabolic  activity  and  growth  may  dctennine  the 
development  of  buds  in  spite  of  the  inhibiting  influence 
of  the  dominant  region.  We  have  seen  how  in  pieces 
of  Tuhularia  stem  the  presence  of  the  wound  at  the 
basal  end  assists  in  establishing  the  new  gradient,  even 
in  spite  of  the  presence  of  the  old  (see  pp.  132-37).  This 
is  a  good  case  of  physiological  isolation  by  the  action  of 
local  factors. 

Further  analytic  investigation  along  these  lines  is 
greatly  needed  to  enable  us  to  determine  the  part  played 
by  the  various  factors  in  different  cases  of  reproduction, 
but  the  mere  observation  of  various  reproductive 
processes — such,  for  example,  as  the  production  of  a  new- 
plant  by  a  strawberry  runner,  after  it  has  attained  a 
certain  length — will  enable  us  to  learn  much  concern- 
ing the  range  of  dominance  and  its  changes  under 
different  conditions. 

The  reduplication  of  parts  in  an  organism,  such  as 
leaves  and  roots  in  the  plant  and  segments  and  various 


196  INDIVIDUALITY  IN  ORGANISMS 

other  parts  in  the  animal,  belongs  in  the  same  category 
with  the  reproductive  processes  which  give  rise  to  new 
whole  organisms.  In  such  cases  physiological  isolation 
may  be  partial  or  with  reference  to  a  specialized  con- 
stituent individual  of  the  organism. 

The  localization  of  reproduction  in  the  individual 
may  be  determined  by  various  other  factors  besides 
distance  from  the  dominant  region.  Some  parts  less 
distant  than  others  may  be  physiologically  isolated 
earlier  because  of  lower  conductivity  of  paths,  or  because 
of  other  correlative  conditions  within  the  organism,  or 
because  of  certain  external  conditions.  In  isolated 
parts  the  least  differentiated  cells  or  regions,  or  those 
with  the  highest  metabolic  rate,  are  likely  to  react  earher 
tnan  others  and  so  determine  the  localization  of  the 
reproductive  process.  Sometimes,  particularly  among 
plants,  in  reproductions  which  occur  with  advancing 
age  or  under  depressing  conditions,  it  is  the  original 
dominant  region  which  separates  from  other  parts  as 
a  smaller  individual  and  so  becomes  the  reproductive 
body,  spore,  or  whatever  it  may  be  called. 

Special  unrecognized  factors  may  play  a  part  in 
certain  cases,  but  it  seems  impossible  to  doubt  that,  in 
general,  agamic  reproduction  in  organisms  results  from 
physiological  isolation  of  parts  of  the  individual.  Indi- 
viduation is  a  physiological  integration  depending  pri- 
marily on  the  dominance  and  subordination  of  part's  in 
relation  to  an  axial  gradient  or  gradients,  and  agamic 
reproduction  is  a  physiological  disintegration  of  this 
unity  which  makes  possible  new  integrations. 

The  fundamental  similarity  in  individuation  and 
reproduction  in  the  lower  animals  and  plants  is  well 


CONCLUSIONS  AND  SUGGESTIONS  197 

illustrated  by  a  comparison  of  certain  corals  with  tlie 
plants.  Wood- Jones'  has  recently  found  from  a  study 
of  living  animals  under  natural  conditions  that  in  the 
staghorn  corals  there  is  a  radially  symmetrical,  apical 
zooid  at  the  tip  of  the  stem  which  gives  rise  by  budding 
to  the  bilaterally  symmetrical,  lateral  zooids,  while 
these  do  not  reproduce  as  long  as  the  apical  zooid  is 
present  and  active.  At  a  certain  distance  from  the 
apical  zooid  one  of  the  bilaterally  symmetrical  zooids 
may  become  radially  symmetrical  and  begin  to  reproduce 
new  zooids  and  so  become  the  apical  zooid  of  a  branch. 
If  the  apical  stem-region  with  the  apical  zooid  is  removed, 
several  branches  may  arise  by  the  transformation  of 
bilateral  into  radial,  reproducing  zooids.  Moreover, 
the  apical  zooid  of  stem  and  branches  remains  young 
indefinitely,  while  the  lateral  zooids  which  do  not 
reproduce  undergo  senescence  and  die.  In  other  corals 
various  degrees  of  composite  individuation  are  found 
to  exist.  The  relation  of  the  dominant  apical  zooid 
to  other  parts  in  the  staghorn  corals  is  very  evidently 
essentially  the  same  as  that  between  the  growing  tip 
and  other  parts  in  plants,  and  it  is  impossible  to  doubt 
that  the  same  fundamental  principle  underlies  and 
determines  the  relation,  not  only  in  these  two  cases, 
but  in  organisms  in  general. 

GAMETIC   REPRODUCTION 

Sexual  or  gametic  reproduction,  with  rare  exceptions 
the  only  reproductive  process  giving  rise  to  whole  new 
organisms    among    the    higher    animals,    is    commonly 

^  F.  Wood-Jones,  Coral  and  Atolls,  London,  191 2,  chaps,  viii,  ix. 


iqS  individuality  in  organisms 

regarded  as  very  different  from  the  agamic  reproductive 
processes.  Actually,  however,  there  are  certain  funda- 
mental similarities  between  the  two  processes.  I  have 
discussed  this  matter  at  some  length  elsewhere/  and 
need  only  review  certain  important  points  here.  The 
evidence  indicates  that  the  gametes,  the  two  cells  which 
unite  in  sexual  reproduction  and  which  in  their  more 
highly  specialized  forms  we  call  egg  and  spermatozoon, 
are  physiologically  subordinate  parts  of  the  body  and 
undergo  differentiation  with  other  parts,  instead  of  being 
composed  of  a  mysterious,  independent  substance,  the 
germ  plasm,  as  Weismann  and  many  others  have  be- 
lieved. Gametic  maturity  occurs  at  a  relatively  ad- 
vanced physiological  age  in  the  organism,  and  the 
gametes,  like  other  parts  of  the  body,  are  physiologi- 
cally old  cells  with  a  low  metabolic  rate  and  are  evi- 
dently approaching  death.  Their  isolation  from  other 
parts  of  the  body  in  those  multicellular  forms  in  which 
complete  isolation  occurs  has  apparently  no  relation 
to  the  range  of  dominance,  but  seems  rather  to  be  asso- 
ciated with  the  completion  of  their  period  of  growth 
and  differentiation.  So  far  as  the  parent  organism  is 
physiologically  concerned,  the  isolation  of  the  sex  cells 
may  be  compared  with  the  casting  off  of  other  old 
cells  which  have  played  their  part  and  are  approaching 
death.  In  many  cases,  however,  the  egg  remains  in 
the  parent  body  until  an  earlier  or  later  stage  of  embry- 
onic development  is  reached,  but  even  in  such  cases 
the  egg,  after  completing  its  developmental  period,  seems 
to  have  little  physiological  relation  to  other  parts  of  the 
parent  body. 

'  Child,  Senescence  and  Rejuvenescence,  19 15,  Part  IV. 


CONCLUSIONS  AND  SUGGESTIONS  jyy 

Except  in  the  case  of  parthenogcnic  eggs,  which 
develop  without  fertiHzation,  neither  of  the  gametes 
undergoes  dedifferentiation  and  a  new  development  Ijy 
itself,  but  in  some  way  their  union,  or  conditions  asso- 
ciated wdth  it,  or  in  various  cases  certain  experimental 
conditions  (''artificial  parthenogenesis"),  initiates  the 
process  of  dedifferentiation  and  rejuvenescence  which 
makes  possible  the  development  of  a  new  individual  and 
a  new  period  of  differentiation  and  senescence.  The 
increasing  metabolic  rate  and  the  loss  of  differentiation 
in  the  early  stages  of  embryonic  development  indicate 
clearly  that  rejuvenescence  is  occurring,  but  sooner  or 
later  the  intake  of  nutrition  results  in  renewed  accumula- 
tion of  substratal  substance  and  senescence  begins  again. 
The  period  of  dedifferentiation  and  rejuvenescence  is 
short,  and  during  most  of  its  development  the  sexually 
produced  organism  is  growing  old. 

As  I  have  endeavored  to  show,  the  development  of 
the  individual  in  gametic  reproduction  is  fundamentally 
the  same  process  as  in  agamic  and  experimental  repro- 
duction. In  most  cases  the  polarity,  i.e.,  the  major 
axial  gradient,  and  in  some  cases  the  minor  gradients, 
are  determined  in  the  eggs  before  embryonic  develop- 
ment begins,  usually,  so  far  as  observation  permits 
definite  conclusions,  by  their  relations  to  the  parent 
body,  but  in  some  of  the  lower  plants  the  major  axis  is 
apparently  determined  after  the  egg  leaves  the  plant- 
body  by  the  direction  or  differential  action  of  light  or 
other  external  factors.  The  point  of  entrance  of  the 
sperm  seems  in  many  cases  among  animals  to  be  a 
factor  in  determining  the  symmetry  gradients,  where 
they  are  not  already  determined.     In  at  least  many 


200  INDIVIDUALITY  IN  ORGANISMS 

plants,  however,   and  doubtless  in  some  animals,   the 
symmetry  gradients  are  determined  in  later  stages. 

From  this  point  of  view  the  chief  difference  between 
agamic  and  gametic  reproduction  is  that  in  the  latter 
the  mere  isolation  of  the  reproductive  body  from  the 
parent  individual  is  not  sufficient  to  start  the  process  of 
dedifferentiation  and  new  development.  The  gametes 
do  not  react  except  under  special  conditions,  because 
they  have  become  so  highly  specialized  and  differentiated 
as  parts  of  the  parent  individual  that  they  are  incapable 
of  such  reaction.  But  when  the  special  conditions  are 
present,  dedifferentiation  begins  and  development  pro- 
ceeds. Certain  eggs  develop  parthenogenically,  and 
these  in  many  cases  are  very  evidently  less  highly  differ- 
entiated than  eggs  which  require  fertilization.  It  is 
probable  that  they  or  some  of  them  represent  a  stage 
in  gametic  development  in  which  the  egg  is  still  capable 
of  reacting  to  isolation  like  the  physiologically  or  physi- 
cally isolated  part  of  the  body  of  Tuhidaria  or  Planaria 
by  undergoing  dedifferentiation  and  a  new  course  of 
development.  If  this  conclusion  is  correct,  these  par- 
thenogenic  eggs  represent  a  condition  intermediate  be- 
tween the  parts  of  the  body  of  lower  forms  which 
undergo  agamic  reproduction  when  isolated  and  the 
more  highly  specialized  gametes  for  which  fertiliza- 
tion is  a  necessary  condition  of  further  activity.  At 
least  many  of  the  eggs  in  which  development  can  be 
initiated  experimentally  by  other  means  than  fertili- 
zation are  apparently  almost  capable  of  natural  par- 
thenogenesis, and  so  are  probably  less  highly  specialized 
than  eggs  which  are  not  susceptible  to  experimental 
treatment. 


CONCLUSIONS  AND  SUGGESTIONS  201 

If  we  accept  this  view,  we  must  regard  gametic 
reproduction  merely  as  a  more  highly  spcciaHzed  form 
of  reproduction  which  occurs  in  more  advanced  life 
or  in  more  highly  differentiated  individuals  than  agamic 
reproduction,  but  which  involves  essentially  the  same 
cycle  of  differentiation  and  senescence,  followed  by 
dedifferentiation  and  rejuvenescence,  the  production  of  a 
new  individual,  and  another  period  of  differentiation 
and  senescence. 

From  this  standpoint  the  egg  and  the  embryo  are 
in  general  the  most  unfavorable  material  that  could  be 
found  for  the  investigation  and  analysis  of  the  processes 
of  reproduction  and  individuation,  for  in  most  cases  the 
gametes  are  formed  in  the  parent  organism  under  C(jn- 
ditions  which  do  not  permit  of  extensive  and  exact  ex- 
perimental control.  Moreover,  they  consist  of  single 
cells,  and  so  cannot  be  divided  experimentally  before  de- 
velopment begins,  and  the  egg  has  usually  attained  a 
certain,  often  a  very  high,  degree  of  individuation  before 
it  is  isolated.  The  agamic  and  experimental  reproduc- 
tions afford  a  much  wider  range  of  control,  and  we  can 
analyze  the  beginnings  of  individuation  there  as  we  can- 
not in  the  egg.  The  only  logical  procedure  is,  in  my 
opinion,  to  interpret  gametic  reproduction,  as  1  have 
attempted  to  do,  on  the  basis  of  our  knowlerlge  of  the 
experimental  and  agamic  processes,  and  not  vice  versa. 
Our  slow  progress  toward  an  adequate  conception  of 
organic  individuality  has  undoubtedly  been  due  in  con- 
siderable part  to  the  fact  that  we  have  confined  our 
attentioTi  so  largely  to  gametic  reproduction,  and  have 
neglected  the  simpler  processes  in  which,  if  anywhere, 
the  key  to  the  problem  is  to  be  found. 


202 


INDIVIDUALITY  IN  ORGANISMS 


HEREDITY,  EVOLUTION,  AND  OTHER  PROBLEMS  FROM  THE 

DYNAMIC  STANDPOINT 

If  the  organism  is  fundamentally  a  specific  reaction 
system  in  which  quantitative  differences  initiate  physio- 
logical individuation,  development,  and  differentiation, 
nothing  can  be  more  certain  than  that  it  acts  essentially 
as  a  unit  in  inheritance.  It  is  the  fundamental  reaction 
system  which  is  inherited,  not  a  multitude  of  distinct, 
qualitatively  different  substances  or  other  entities  with 
a  definite  spatial  localization.  Development  is  not  a 
distribution  of  the  different  qualities  to  different  regions, 
but  simply  the  realization  of  possibilities,  of  capacities 
of  the  reaction  system.  The  process  of  realization  differs 
in  different  regions  because  the  conditions  are  differ- 
ent. Neither  characters  nor  factors  as  distinct  entities 
are  inherited,  but  rather  possibilities,  which  are  given 
in  the  physico-chemical  constitution  of  the  fundamental 
reaction  system,  but  not  necessarily  localized  in  this 
or  that  part  of  it. 

The  fact  that  in  the  past  investigation  of  inheritance 
has  been  almost  entirely  limited  to  the  special  aspects 
of  heredity  and  development  connected  with  gametic 
reproduction  has  contributed  very  largely  to  delay  our 
progress  and  limit  and  distort  our  conceptions  of  the 
processes  cf  inheritance.  This,  the  most  highly  special- 
ized form  of  reproduction,  is  the  most  unfavorable 
point  of  attack  upon  the  problems  involved,  for  the 
possibilities  of  control  of  the  earlier  stages  of  individu- 
ation are  narrowly  limited,  and  many  factors  which 
are  not  really  essential  to  reproduction  and  develop- 
ment are  characteristically  present  in  this  reproductive 
process. 


CONCLUSIONS  AND  SUGGESTIONS  203 

The  process  of  inheritance  is  involved  to  exactly 
the  same  extent  in  the  reconstitutional  development 
of  a  new  individual  from  a  piece  of  Tuhularia  stem  or 
of  the  planarian  body,  or  in  the  formation  of  a  new  grow- 
ing tip  from  the  differentiated  cells  of  a  leaf  (Figs.  38,  39), 
from  callus  tissue  (Fig.  40),  or  from  any  other  part  of 
the  plant,  as  it  is  in  the  reproduction  of  a  new  individual 
from  the  egg,  with  or  without  fertilization,  in  any  of 
these  forms.  The  simple  agamic  and  experimental  re- 
productions, moreover,  afford  very  much  greater  pos- 
sibilities for  the  analysis  and  control  of  the  processes 
and  mechanism  of  inheritance  and  develoj^ment  than 
gametic  reproduction.  Any  adequate  conception  of  in- 
heritance and  development  must  be  based  upon  ana- 
lytic investigation  of  these  simple  reproductions  and 
synthesis  of  the  results,  and  it  must  interpret  inheritance 
in  gametic  reproduction  in  terms  of  the  simpler  processes. 
Continued  sexual  breeding  and  hybridization  under 
controlled  conditions  and  with  pedigreed  individuals  has 
contributed  much  and  undoubtedly  will  contribute 
further  toward  the  solution  of  certain  special  problems 
of  inheritance,  and  also  affords  results  which  possess  a 
statistical  value,  but  this  method  of  procedure  alone 
can  never  carry  us  very  far  toward  the  solution  of  the 
fundamental  problem  of  inheritance.  The  key  to  this 
problem  also  will  be  found  in  the  simpler  reproductive 
processes. 

If  the  organism  is  a  unit  in  inheritance  and  develop- 
ment we  must  expect  to  find  that  so-called  "acquired 
characters"  may  be  impressed  on  the  organism  to  such 
a  degree  that  sooner  or  later  the  reaction  system  may 
give  rise  to  these  characters  without  the  action  of  the 


204  INDIVIDUALITY  IN  ORGANISMS 

particular  external  factor  which  originally  produced 
them.  The  reaction  of  the  organism  to  a  sufficient 
local  excitation  is  not  simply  a  local  reaction,  but  a 
reaction  more  or  less  of  the  whole  organism,  and  we  know 
that  in  the  case  of  many  physiological  reactions  the 
repetition  of  the  reaction  in  response  to  repeated  external 
excitation  alters  the  reaction  system  so  that  response 
occurs  more  readily  or  more  rapidly  or  with  a  lower 
intensity  of  stimulus.  We  say  that  the  irritability  of  the 
protoplasm  is  increased,  its  "threshold"  for  stimulation 
is  lowered,  etc.  If  this  change  goes  far  enough  the 
reaction  may  occur  in  the  absence  of  the  external  factor 
which  first  produced  it,  simply  because  the  condition 
or  constitution  of  the  protoplasm  has  been  so  altered 
by  the  repetition  of  the  reaction  that  it  occurs  auto- 
matically when  any  condition  determines  a  sufficiently 
high  metabolic  rate  in  the  reaction  system.  The 
"inheritance  of  acquired  characters"  then  belongs  in 
the  same  general  category  as  the  increase  in  irritability 
resulting  from  repeated  excitation,  but  it  may  in  many 
cases  require  thousai>ds  or  hundreds  of  thousands  of 
generations  before  a  condition  approaching  auto- 
maticity  in  its  production  is  attained.  In  the  face 
of  the  physiological  facts  it  is  difficult  to  understand 
how  biologists  can  continue  to  maintain  the  distinction 
between  soma  and  germ  plasm,  and  to  content  them- 
selves with  the  assertion  that  natural  selection  is  ade- 
quate to  account  for  adaptation  in  the  organic  world. 
If  the  organism  is  in  any  sense  a  dynamic  entity,  then 
its  evolution  must  be  a  reaction  determined,  on  the 
on^.  hand,  by  its  physico-chemical  constitution,  and 
on  the  other,  by  its  relation  with  the  external  world, 


CONCLUSIONS  AND  SUGGESTIONS  205 

and  its  adaptations  are  simply  special  features  of  this 
relation. 

Evolution  is  not  directly  concerned  with  morpho- 
logical characters,  but  with  the  physico-chemical  con- 
stitution of  the  reaction  system,  and  so  with  the  rate 
and  character  of  its  reactions  and  the  conditions  under 
which  they  occur.  I  have  called  attention  elsewhere' 
to  the  resemblance  between  the  progress  of  evolution 
and  the  progress  of  senescence  and  development  in  the 
individual,  and  have  suggested  that  evolution,  like 
senescence  and  other  processes  in  nature,  may  be  essen- 
tially a  change  from  a  less  stable  to  a  more  stable  condi- 
tion in  the  dynamic  reaction  system  which  constitutes 
the  organism. 

The  significance  of  this  dynamic  conception  of  the 
organism  for  various  other  biological  problems  will  be 
apparent  without  further  discussion,  and  I  beheve  it 
may  possess  a  certain  significance  for  certain  problems 
of  comparative  psychology  and  sociology.  It  is  at 
least  a  matter  of  some  interest  to  be  able  to  trace  the 
fundamental  identity  in  individuation  from  the  simple 
unicellular  organism  to  the  highest  plants  in  the  one 
direction  and  to  conscious  man  in  the  other,  and  to  show- 
that  the  growing  tip  of  the  plant  and  the  brain  of  man 
have  something  in  common.  Moreover,  to  find  the 
same  principle  of  individuation  in  the  egg  and  in  the 
adult  organism  and  again  in  the  single  nerve  cell  and 
its  fiber  is  at  least  highly  suggestive.  The  recognition 
of  the  fact  that  individuation  in  the  organism  is  a  rela- 
tion of  dominance  and  subordination  of  parts  removes 
much  of  the  difficulty  in  accounting  for  the  high  degree 

^  Child,  Senescence  and  Rejuvenescence,  1915,  pp.  144,  i03.  463-65. 


2o6  INDIVIDUALITY  IN  ORGANISMS 

of  definiteness  and  the  constancy  of  character  of  the 
developmental  processes  and  other  activities  of  living 
things.  It  also  has  a  certain  bearing  upon  the  problem 
of  the  origin  of  individuations  whose  component  parts 
are  human  beings  or  groups  of  human  beings.  Between 
the  organic  individual  and  the  state  there  is,  from  this 
point  of  view,  a  real  analogy,  for  control  or  government 
is  tjie  essential  feature  in  the  individuation  in  both,  and 
the  relations  are  in  certain  respects  similar  in  both 
cases.  It  is  not  a  mere  fanciful  analogy  to  conceive 
the  organism  as  a  state  or  the  state  as  an  organism, 
since  both  are  dynamic  individuals  and  some  degree 
of  dominance  or  government  exists  in  both.  These 
suggestions  are  an  indication  of  some  of  the  broader 
bearings  of  the  dynamic  conception  of  the  organic 
individual,  but  discussion  along  these  lines  must  be 
postponed. 

In  conclusion  it  is  perhaps  permissible  to  call  atten- 
tion to  the  simplification  and  unification  of  viewpoint 
which  this  conception  accomplishes.  The  separation  of 
morphological  from  physiological  investigation  and 
thought,  particularly  in  zoology,  which  followed  the 
acceptance  of  the  theory  of  evolution,  and  the  fact  that 
the  morphologists,  rather  than  the  physiologists  or 
biochemists,  have  chiefly  concerned  themselves  with  the 
great  problems  of  heredity,  development,  and  evolution, 
have  brought  it  about  that  biological  theory  in  these 
fields  has  been  to  some  extent  a  world  apart.  While 
proclaiming  their  acceptance  of  the  mechanistic  or 
physico-chemical  conception  of  life,  the  theorists  of  this 
group  and  their  followers  have  not  only  made  but  few 
attempts    to    apply    physico-chemical    conceptions    to 


CONCLUSIONS  AND  SUGGESTIONS  207 

the  organism,  but  have  often  decried  the  value  of  such 
attempts.  It  is  still  true,  therefore,  to  a  large  extent 
that  to  grasp  these  theories  we  must  enter  a  new  world 
of  symbols,  which  only  too  often  appear  to  have  no 
resemblance  or  relation  to  any  other  symbols  commonly 
in  use  in  scientific  thought.  When  we  have  become 
famihar  with  our  new  world,  we  can  perform  marvelous 
feats  with  its  symbols  and  fill  our  pages  with  fonnulae  of 
gametic  constitution  or  what  not,  but  so  far  as  any  real 
connection  between  this  world  and  the  other  world  of 
science  is  concerned,  such  theories  and  their  symbols 
leave  us,  at  least  in  most  cases,  eaxctly  where  we  were 
at  the  beginning.  We  can  discuss  the  topographic 
location  of  hereditary  factors  in  the  chromosome,  and 
we  can  arrange  them  in  any  way  necessary  to  account 
for  the  observed  facts.  In  fact,  we  can  invent  symbols  to 
describe  development  or  any  other  process  in  the  organ- 
ism. But  some  of  the  discussions  which  have  to  do 
with  these  static,  morphological  symbols  remind  us 
irresistibly  of  that  old  problem  of  the  angels  and  the 
needle  point. 

Being  entirely  unable  to  find  any  degree  of  intellec- 
tual satisfaction  in  those  static  conceptions  of  the 
organism  which  seem  to  have  no  relation  to  anything 
else  in  the  world  and  which  raise  many  questions  but 
answer  none,  and  being  forced  by  my  own  experimental 
investigations  to  conclusions  very  different  from  these, 
I  have  attempted  to  apply  dynamic  conceptions  to  cer- 
tain biological  problems,  with  the  results  which  have 
been  considered  in  the  preceding  pages.  Whatever 
other  value  the  dynamic  viewpoint  may  i)ossess,  it 
serves  as  a  basis  for  the  synthesis  and  ordering  of  many 


208 


INDIVIDUALITY  IN  ORGANISMS 


facts  in  various  fields  whi  -h  heretofore  have  seemed  to 
have  little  or  nothing  in  common,  and  I  think  we  may- 
say  that  it  aids  in  bringing  certain  aspects  of  biology  at 
least  within  hailing-distance  of  physico-chemical  con- 
ceptions. 


INDEX 


Note. — References  give  the  number  of  the  page  on  which  the  matter  referred  to  begins. 


Anophthalmic   form   in   Planaria, 
io6,  141. 

Axis,  organic:  occurrence  of,  8; 
apical  and  basal  ends  of,  10;  ter- 
minology of,  19;  simplest  form 
of,  35;  susceptibility  gradients 
in  relation  to,  53,  60;  independ- 
ence of  apical  region  of,  96,  113; 
dominance  of  apical  region  of, 
102;  control  of  space  relations 
in,  128;  experimental  oblitera- 
tion and  determination  of,  142; 
as  resultant  of  two  pre-existent 
axes,  164;  quantitative  charac- 
ter of,  167,  185;  time  of  deter- 
mination of  in  egg  and  embryo, 
199.  5ee  a/50  Dominance;  Gra- 
■  dients;  Individual;  Polarity; 
Symmetry 

Begonia,  adventitious  buds  in,  S3. 

Biaxial  forms:  in  Tubular ia,  97, 
133;  in  Planaria,  99,  117;  ex- 
perimental transformation  of,  in 
Corymorpha,  144;  experimental 
determination  of,  in  Planaria, 
149.  See  also  Axis;  Gradients; 
Intlividual 

Conductivity:  in  relation  to  trans- 
mission, 40;  increase  in,  during 
development,  150;  in  relation  to 
physiological  isolation,  194.  See 
also  Transmission 
Corals,  individuation  in,  197 
Correlation,  physiological:  differ- 
ent kinds  of,  4,  27;  occurrence 
of  transportative,  26,  44,  170; 
conditions  determining  trans- 
portative,  26,  170,  172.  Sec  also 
Axis;  Dominance;  Gradients; 
Individual;  Transmission; 
Transportation 


Corymorpha:  dcscri[)tion  of,  92; 
metabolic  gradients  in,  132;  ob- 
literation and  determination  of 
gradients  in,  142 

Cr^'stal,  compared  with  organic  in- 
dividual, 24 

Cyclamen  persicum,  dominance 
and  subordination  in  leaf  of,  1 56 

Dedifferentiation:  in  agamic  re- 
production, 7, 9, 91 ;  in  formation 
of  adventitious  indi\iduaI.^  in 
plants,  83;  in  reconstitution  of 
Planaria,  109;  capacity  for,  in 
lower  and  higher  animals,  120; 
in  embryonic  development,  199. 
See  also  DitTerentiation 

Differentiation:  occurrence  of,  6; 
orderly  character  of,  7;  different 
degrees  of,  in  eggs  and  embryos, 
121;  in  relation  to  metabolic 
gradient,  171;  in  relation  to 
metabolic  rate,  183,  190.  See 
also  Dedifferentiation 

Dominance,  physiological:  origin 
of,  36,  181;  in  relation  to  meta- 
bolic gradients,  37,  88,  171; 
range  of,  45,  127,  133,  i3>^.  i40, 
162,  172;  in  experimental  re- 
production of  Tubular  ia,  102, 
133;  in  experimental  reproduc- 
tion of  Planaria,  102,  114;  of 
apical  region  in  plants,  104,  152; 
experimental  control  of,  in  Tu- 
bularia,  134;  exjH'rimental  ob- 
literation and  tlelcrmination  ol, 
142;  extension  of,  durin^^  devel- 
opment, 149;  in  relation  to  size 
of  individual,  151,  193;  '"  rela- 
tion to  adventitious  buil.--  in 
plants,  154;  of  growing  tip  in 
conifers,  154;  threction  of.  in 
plants,  155;  inleaf  of  C>7amf»i, 


209 


2IO 


INDIVIDUALITY  IN  ORGANISMS 


156;  in  root  system,  157;  nature 
of,  170;  in  the  neuron,  173; 
decrease  of,  in  relation  to  physio- 
logical isolation,  193;  in  corals, 
197,  See  also  Gradients;  Indi- 
vidual; Isolation 

Entelechy,  23,  137,  184 

Evolution:  increasing  stability  of 
order  in,  6;  in  relation  to  envi- 
ronment, 204;  as  an  equilibra- 
tion process,  205 

Fertilization,  199 

Fission;  in  Stenostomum,  79;  in 
Planar ia,  92,  140,  141 

Frog,  developmental  gradient  in 
early  development  of,  66 

Ginkgo:  developmental  gradient  in 
embryo  of,  73;  formation  of 
growing  tip  of,  77 

Gradients,  developmental:  in  re- 
lation to  metabolic  gradients, 
65;  in  early  embryo  of  frog,  66; 
in  flatworm,  67;  in  chick  em- 
bryo, 69;  in  relation  to  rate  of 
growth,  72;  in  embryo  of  moss, 
73;  in  embryo  of  Ginkgo,  73; 
in  plant  axes,  73;  in  bilater- 
ally symmetrical  plants,  77;  in 
agamic  reproduction  of  Pen- 
naria,  79;  in  reconstitution  of 
Planaria,  81;  in  Metzgeria,  83; 
in  adventitious  buds  of  Bego- 
nia, 83;  in  buds  on  callus,  86. 
See  also  Gradients,  metabolic 

Gradients,  metabolic:  origin  of^ 
29,  181;  as  simplest  expression 
of  order,  35,  187;  in  relation 
to  physiological  dominance  and 
subordination,  36,  170;  inter- 
ference between,  39,  178;  effect 
of,  on  protoplasm,  40;  inherit- 
ance of,  41,  182;  as  basis  of 
qualitative  differences,  42;  dem- 
onstration of,  as  susceptibility 
gradients,  52;  in  animals,  53, 
59;    in  Stentor,  55;    in  starfish 


egg,  56;  in  parts  and  organs,  57; 
demonstration  of,  by  differen- 
tial inhibition,  58;  in  relation  to 
axes,  60;  in  plants,  61;  as 
gradients  in  carbon-dioxide  pro- 
duction, 62;  in  neuron,  62,  151, 
173;  in  relation  to  differences  in 
electrical  potential,  63;  dem- 
onstration of,  by  differential 
staining,  64;  in  relation  to 
developmental  gradients,  65, 
79;  in  experimental  repro- 
duction in  Marchantia,  86,  165; 
in  Tubularia,  91;  in  agamic 
reproduction  of  Planaria,  93; 
independence  of  apical  regions 
of,  96;  in  reconstitution  of 
Tubular  ia,  130;  control  of 
length  of,  in  Planaria,  140;  ex- 
perimental obliteration  and  de- 
termination of,  142;  localization 
as  resultant  of  different,  164; 
problem  of  different  kinds  of, 
178;  relation  of,  to  inhibition, 
178,  See  also  Axis;  Domi- 
nance; Individual 

Growling  tip:  as  feature  of  plant 
individual,  73;  in  relation  to  de- 
velopmental gradients,  74;  in 
adventitious  individuals,  83;  in 
relation  to  range  of  dominance, 
150;  dominance  of,  in  plants, 
152;  localization  of,  as  resultant 
of  different  axes,  165;  self- 
determination  in,  189;  condi- 
tions determining  character  of, 
190 

Harenactis,  control  of  reconstitu- 
tion in,  146 

Head-determination,  in  Planaria, 
III 

Headless  form;  in  Planaria,  106; 
conditions  determining,  118, 
141 

Head-frequency:  in  pieces  of  Pla- 
naria, 108;  experimental  altera- 
tion of,  108;  interpretation  of, 
119;  relation  of,  to  metabolic 
rate,  184 


INDEX 


211 


Individual,  organk:    fundamental 
characteristics  of,  2;    nature  of 
unity   in,   3,   48,    175;     various 
theories  of,  3,  22;   character  of 
order  in,  8,  17,  35;  reproduction 
in  relation  to,  12;    terminology 
of,    18;     comparison    of,    with 
social  individual,    21,    26,   206; 
formulation  of  the  problem  of 
the,  29;   dynamic  conception  of 
the,  29,  88,  172;  as  one  or  more 
metabolic    gradients,    40,    170; 
limitation  of  size  of,  45,  47,  151; 
as  result  of  relation  between  pro- 
toplasm and  environment,  49; 
origin  of  adventitious,  in  plants, 
Ss,  154,  194;  size  of,  in  relation 
to    range    of    dominance,    151; 
fundamental  reaction  system  of, 
188;    difference   between  plant 
and  animal,  189;   in  relation  to 
inheritance,  202;  significance  of 
dynamic  conception  of,  205,  See 
also   Axis;     Dominance;     Indi- 
viduality; Individuation 
Individuahty :    different  kinds  of, 
48;  superficial  origin  of  organic, 
49.     See  also  Axis;  Dominance; 
Individual;  Individuation 
Individuation:    in  Amoeba  proto- 
plasm,  6;    in  experimental  re- 
production, 14;    nature  of,  41, 
48;    in  "rings"    in   Harenactis, 
146;      conditions     determining 
low  degree  of,  in  plants,  189;  in 
corals,  197;    degree  of,  in  egg, 
201;    fundamental  identity  of, 
in    organisms,     205.     See    also 
Axis;    Dominance;    Individual; 
Individuality 
Inheritance:    of  metabolic  gradi- 
ents, 41,  182;   in  relation  to  or- 
ganic individual,  202;  in  relation 
to   different    reproductive   pro- 
cesses, 202;    of  "acquired  char- 
acters," 204 

Inhibition:  of  head-formation  in 
Flanaria,  112,  141;  in  reconsti- 
tution  of  Tiibulana,  135;  of 
growing  shoots  in  plants,  153; 
in    apical    direction    in    plants, 


155;  by  leaves  in  plants,  156; 
of  root-formation  by  roots,  151;; 
non-specific  character  of,  "  in 
plants,  168;   nature  of,  178 

Irritability:  increase  of,  by  re- 
peated excitation,  33,  204;  gra- 
dient of,  34,  180 

Isolation,  physiological:  condi- 
tions determining,  45;  effect  of, 
46;  infrequency  of,  in  higher 
forms,  47;  in  agamic  reproduc- 
tion in  Tubular ia,  92;  in  agamic 
reproduction  in  Flanaria,  94, 
experimental,  in  Tubular  ia,  155; 
experimental,  in  Flanaria,  141; 
experimental,  in  plants,  152;  as 
the  basis  of  agamic  reproduc- 
tion, 192;  diff'erent  conditions 
determining,  193;  in  relation  to 
repudiation  of  parts,  195.  Su 
also  Dominance 

Lumbriculus,  experimental  control 
of  reconstitution  in,  :i8 

Marcliantia:  experimental  repro- 
duction in,  86;  localization  in, 
as  resultant  of  different  axes, 
165 

Metabolism:  characteristics  ot, 
15;  relation  of,  to  protoplasm, 
16;  susceptibility  in  relation  to, 
51 ;  increase  in  rate  of,  after  sec- 
tion in  Flanaria,  no;  rate  of,  in 
relation  to  differentiation,  183, 
190;  rate  of,  in  relation  to  sta- 
bihty  of  structure,  191.  See 
also  Gradients;  Individual;  Ir- 
ritability 

Mctzgeria,  agamic  reproduction  in, 

83 
Moss,  developmental  gradient  in 

embryo  of,  y^ 

Nervous  system;  in  relation  to 
metabolic  gradients,  40,  Oi,  175; 
superficial  origin  of,  4();  meta- 
bolic gradient  in  cells  of,  O2,  i  sr, 
173;  independent  fc.rmation  of, 
in  reconstitution,  114;  supposed 


212 


INDIVIDUALITY  IN  ORGANISMS 


formative  influence  of,  119,  176; 
self-determniation,  of,  in  devel- 
opment, 120,  188;  extension  of 
dominance  in,  151;  dominant 
region  of,  175;  Tunctional  domi- 
nance of,  176;  possible  nature  of 
inhibition  in,  180;  in  relation  to 
fundamental  reaction  system, 
188;  animal  organism  in  rela- 
tion to,  i8g;  possibility  of  de- 
differentiation  in,  191.  See  also 
Conductivity;  Transmission 

Organization:  theories  of,  22;  as 
a  condition  of  chemical  correla- 
tion, 26;  not  the  basis  of  organic 
individuality,  41;  in  relation  to 
minimal  size  in  reconstitution, 
124;  in  relation  to  experimental 
conditions,  1S4 

Parthenogenesis,  igg,  200 

Pennaria,  developmental  gradients 
in  agamic  reproduction  of,  79 

Flanaria  dorotocephala:  suscepti- 
bility gradients  in,  52;  develop- 
mental gradients  in  experimental 
reproduction  of,  81;  agamic  re- 
production of,  92;  experimental 
reproduction  in  short  pieces  of, 
99;  dominance  and  subordina- 
tion in,  102;  reconstitution  in, 
105;  different  forms  of  head  in, 
106;  head-frequency  in  experi- 
mental reproduction  of,  108;  ex- 
perimental control  of  head- 
frequency  in,  108;  control  of 
range  of  dominance  in,  138;  de- 
termination of  biaxial  forms  in, 
149;  extension  of  dominance  in, 
149;  localization  as  resultant  of 
diffei»unt  axes  in,  164 

Planaria  maculafa,  head-frequency 
in  reconstitution  of,  113 

Polarity:  occurrence  of,  8;  theo- 
ries of,  28;  obliteration  of,  in 
experimental  reproduction,  100; 
origin  of,  181;  nature  of,  1S2. 
See  also  Axis;  Dominance; 
Gradients;  Individual 


Poplar,  development  of  buds  on 
calculus  in,  86 

Protoplasm:  in  relation  to  metabo- 
lism, 16;  as  a  metabolic  prod- 
uct, 17;  metabolic  gradients  in, 
34;  differentiation  of,  in  rela- 
tion to  metabolic  gradients,  171; 
effect  of  quantitative  external 
factors  on,  186 

Reconstitution:  in  relation  to 
metabolic  gradients  in  Planaria, 
81;  independence  of  apical  re- 
gion in,  97;  dominance  and  sub- 
ordination in,  102;  the  process 
of,  in  Planaria,  105;  limiting 
factors  in,  117;  progressive  limi- 
tation of,  in  animals,  120;  in 
embryonic  stages,  121;  propor- 
tional relations  of  parts  in,  122; 
limit  of  size  in,  1 24;  of  hydranth 
in  Tubularia,  128;  in  long  pieces 
of  Tubularia,  132;  of  "rings"  in 
Harcnactis,  146.  See  also  Indi- 
vidual 

Rejuvenescence:  nature  of,  46,  90; 
in  reconstitution  of  Planaria,  89; 
in  posterior  zooids  of  Planaria, 
94;  capacity  for,  in  lower  and 
higher  animals,  120;  in  relation 
to  physiological  isolation,  193; 
in  early  embryonic  develop- 
ment, 199.     See  also  Senescence 

Reproduction,  agamic:  occurrence 
of,  12,  89;  of  parts,  13,  195;  in 
relation  to  physiological  isola- 
tion, 45,  192;  in  Pennaria,  79; 
in  Stenostommn,  79;  in  Mets- 
geria,  83;  of  adventitious  buds 
in  Begonia,  83;  in  Tubularia, 
92;  in  Planaria,  92;  different 
conditions  determining,  193; 
localization  of,  196;  difference 
between  and  gametic,  198.  See 
also  Isolation;  Reproduction, 
gametic 

Reproduction,  experimental:  sig- 
nilicance  of^  14,  88;  in  Plana- 
ria, 81,  105;  in  poplar,  86;  in 
Marchantia,  86,   165;    in  short 


INDEX 


213 


pieces  _  of  Tubnlaria,  97;  in 
short  pieces  of  P/awana,  gg;  in 
long  pieces  of  Tubnlaria,  132; 
in  plants,  152;  in  relation  to 
shape  of  piece  in  Planaria,  114. 
See  also  Reconstitution;  Repro- 
duction, agarnic,  gametic 

Reproduction,  gametic:  occur- 
rence of,  13;  nature  of,  47,  igS; 
difference  between,  and  agamic, 
200.  See  also  Reproduction, 
agamic,  experimental 

Roots  of  plants :  as  subordinate  in- 
dividuals, 104,  162;  dominance 
and  physiological  isolation  in, 
157;  relation  of,  to  other  parts 
of  plant,  i5g;  conditions  deter- 
mining formation  of,  162 

Sea-urchin,  control  of  proportions 
in  larva  of,  58 

Senescence:  nature  of,  46,  go;  in 
relation  to  self-maintenance  of 
parts,  177;  in  relation  to  ga- 
metic reproduction,  igS;  in  re- 
lation to  evolution,  205.  Sec  also 
Rejuvenescence 

Starfish:  axial  relations  in,  8;  sus- 
ceptibilit}'  gradient  in  egg  of,  56 

Stenosiomum:  agamic  reproduc- 
tion in,  7g;  extension  of  domi- 
nance in,  150 

Stentor,  susceptibility  gradient  in, 
55 

Subordination,  physiological:  ori- 
gin of,  36;  of  basal  to  apical 
levels  in  Planaria,  102,  115;  of 
basal  to  apical  levels  in  plants, 
104.     See  also  Dominance 

Susceptibility:  in  relation  to  me- 
tabolism, 51;  gradients  of,  52; 
gradient  of,  in  Stentor,  55;   gra- 


dient   of,    in    starfish    egg,    56; 
gradient  of,  in  plants,  6i 

Symmetry:  occurrence  of,  8;  of 
starfish,  8;  suscei)iibi!itv  grnrii- 
ents  in  axes  of,  57;  bihiteral  in 
certain  plants,  77,  86;  in  "rings" 
m  Ilarenactis,  148;  in  conifers. 
'^SS\  origin  of,  181;  significance 
of  experimental  alterations  of, 
182;  conditions  determining, 
igg.  See  also  kias;  Gradients; 
Polarity 

Teratomorphic  form,  in  Planaria, 
106,  141 

Teratophthalmic  form,  in  Pla- 
naria, 106,  141 

Transmission:  decrement  in,  29, 
32,  45,  47,  150,  172,  173;  nature 
of,  31,  44,  177;  in  relation  to 
conductivity,  40;  possibility  of 
differentkindsof,  43,  178;  range 
of,  45;  increase  in  range  of,  dur- 
ing development,  i4g;  in  rela- 
tion to  dominance,  170.  See 
also  Conductivity 

Transportation:  occurrence  of,  in 
organisms,  4,  27;  conditions  de- 
termining, 26,  170,  172;  from 
roots  of  plants,  161.  See  also 
Correlation 

Tubnlaria:      description     of,     91; 
agamic     reproduction     in,     92; 
experimental     reproduction     ii. 
short  pieces  of,  97,  133;    domi- 
nance in  experimental  reproduc 
tion  of,  102;   control  of  distance 
relations    in    reconstitution    ot. 
128;   reconstitution  of  hydranlh 
of,  128;    reconstitution  in  long 
pieces  of,  132;    range  of  donii 
nance  in,  133;    inhibition  in  re- 
constitution of,  135 


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